Alright, full disclosure — I’m a Mac guy. 
It’s not that I have any fisticuff issues with Qlab on PC’s, but I’ve had more success on the Mac, and QLab is the industry standard for most sound designers. 
Anyway, QLab 1 came out May 25th, 2007 for the Mac (source). It’s come a long way since 2007, and is currently on version 3.1.7. Thank Dionysus.


What’s So Special About QLab 3?
Well, a lot really. QLab lets you interface with complex audio systems in a pleasantly simple way. 
Basically, you hook an audio interface (such as a Saffire Pro 40) into your computer — your interface into your audio mixer — and then your mixer to your speakers. Again that was: Computer — Interface — Mixer — Speakers. Boom, you have a series of speakers that work independently with any channels you route within QLab.  
You want a car to pull up stage right? Done.
You want that thunder to come from your hanging speakers and subs? Done.
You want to blast some frackin’ Flight of the Valkyries through everything!? Done, AND done. Well then you should learn how to use Qlab
Types Of Cues

CueLab doesn’t just do audio. I mean, it could, but that’d be boring. It allows you to program projections, microphones, cams, and midi. It can basically do everything on the control end, even down to programming the Qlab software with new scripts. However, we won’t delve that deep today. Another day. Not today.
Head’s up, I’m just gonna go left to right on these cue lab types highlighting the ones that will be best to explain for early use.
Group – The group cue allows you take a series of other cues, and hold them within a folder. This is a great option for containing a complex storm, preshow music, or even dense sequences that would otherwise clutter up your space.
Groups have four modes: start first child (cue) and enter into the group, start first child (cue) and go to next cue, start all children (cues) simultaneously, or start random child (cue) and go to next cue. 
Audio – The audio cue software is what you’d expect it to be: a cue that plays audio. You can simply drop the file directly into the program, and it’ll pretty much be ready to go. You can can enter the Time & Loops tab to set the length of the cue, or design loops.
You can go to the Devices & Levels tab to tell QLab which audio interface you’d like the cue to play from, and which channel you would like to route it out. (That’s how you independently send a cue to a speaker.) The Trim tab allows overall audio volume adjustment to the cue that will not be altered by fades.
Finally, there is the Audio Effects tab — which is a new feature to QLab 3 — where you can add effects such as EQ, reverb, or delay to your cue. Believe me, the audio effects tab is a game changer for ease of use, and time saving.
Mic – The mic cue allows you to take a microphone feed from your  qlab audio interface, process it in QLab, and then route it as you’d like. You can mute microphones, add effects, and route them to their final destination.
On release, the mic cue was pretty buggy, and had some serious delay issues, but QLab has been hard at work resolving the issue. It basically takes the place of midi control to a sound board if you opt for it.
Video – The video cue software is primarily for displaying images, or playing videos. It has a lot of the same flexibilities as the Audio cue. Major differences include being able to alter the scale, opacity, and position. Just as you would route audio to different interfaces, you can route the video to different projectors or screens. Pretty snazzy eh?
Cam – The cam cue let’s you take a camera which has been interfaced with the computer, and project it to any screen, or monitor just as you’d do with a video cue.
Title – The title cue allows the user to design a… well… title. From there you can alter it just as you would a video cue.
Fade – The fade cue is one of the big work horses of QLab. It not only allows you to fade out audio however you’d like, but also fades video, and can be used to automate effects, or alter properties for video. 
OSC – The OSC cue starts to get a bit complex. Basically, it can trigger a number of cues a number of ways whether it be to tell a cue within QLab to fire, or ordering a piece of gear to fade. A book could be written covering OSC uses. We won’t worry about going any deeper today. 
MIDI – The midi cue is one of the simplest forms of control you can employ with QLab. All you have to do is set up a midi connection either internally in the computer to control something like MainStage, or externally to control something like a sound board, lighting console, or even another computer.
This is great for making one computer control lights, sound, and projections. I’ll be writing an article on this soon, so keep up with the posts!
MIDI File – The midi file cue is pretty sweet in that you can load a .midi file into QLab, and tell it to play through an interfaced instrument. Have I ever felt the need to do this… no. Will I ever use it… well, I could become an underground digital media artist who wants to take the world by storm eventually. Right? No? Anybody? Moving on…
Timecode – I’ve never used it, but timecode allows you to set a sync capable timecode that all connected systems can operate on. Basically, all the machines connected will work as a unit triggering cues that have all been automated together on a timeline. Again, this get’s complex quick, and it’s not needed for our basic uses.
Play – A play cue… plays an assigned cue. I tend to use the play cue at the end of a preshow music group to retrigger the beginning of the playlist just in case we need to fill more time. 
Stop – A stop cue stops whatever cue it’s assigned to. They’re great for hard cuts where you don’t a want a fade. I often do a stop cue with an auto-continue into a record scratch. The effect works every time. Rrrrrrvvvvp!
Pause – The pause cue pauses the assigned cue. From there you can follow with a play cue to start up where you left off. It’s a nice effect for when someone pauses a stereo onstage.
Load – The load cue essentially arms whatever cue it’s assigned to. This matters because some files are very large, and the computer has to have them geared up in order to fire correctly. I’m personally a fan of having auto-load set up in preferences.
Reset – The reset cue takes a cue that has been partially played and resets it. I’ve never really had a use for it because there are other ways to do this some thing. 
Devamp – The de-vamp cue is awesome if you know what you’re doing with it. Say you have a musical with canned music, and you need it to vamp for dialogue, but want to be able to continue when the actor finishes the talking. Simply set a loop in the audio cue, that will loop infinitely.
From there assign the audio track to a de-vamp cue, and fire it when you are ready to move on in the song. You can do this as many times as you like in a tune. It’s extremely handy for making transition music time out perfectly every time.
Go To – The go to cue will take your session selection wherever you assign it when fired. Again, with theatre I don’t find much use for the Go To cue, but I’m sure there are some neat uses once you get into scripting. 
*I’m skipping target, arm, disarm, and wait. I haven’t used them in the theatre. 
Memo – The memo cue allows you to leave yourself a message, so you don’t forget to do something important like… cut the lights to black from a wall control. Remember to eat a sandwich. Remember that you have 10 minutes till the next cue, so you can… I dunno… chill. You get the idea.
Script – The script cue is the sandbox of QLab. You basically have a wide open box of possibilities much like when a web design platform gives you an html box. If you know code, you can just tear it up with the script box (positive kind of tearing it up). Here’s a link to the QLab Script Wiki. I’ll just caution that if you don’t know what you’re doing, you can do some damage to your session, so tread carefully.
 
Other Perks
Continues and Follows – Continues and follows can be added to any cue through the basics menu, or directly on the cue screen to the right of the post wait. 
The first click changes the continue to a simple auto-continue. That means that when the cue is fired, the following cue will also be fired simultaneously. 
The second click changes the auto-continue to an auto-follow. An auto-follow means that the initial cue will play out entirely, then automatically trigger the next cue. This is perfect for building pre-show music playlists, just be sure to include a play cue at the bottom that loops back to the top of the list, and also make sure the group is set to the correct option. Usually I recommend fire first child, and go to next cue. 
Pre and Post Wait – Within the standard cue window, you can add a pre or post wait. A pre wait tells the cue that once it is fired, it must wait the set amount of time before actually firing. With a post wait, the cue will fire, and then wait to continue on for the set amount of time. Waits are often used in conjunction with auto-continues and auto-follows to time out long sequences of cues.

Internal Volume Automation – When you go to the Times & Loops tab of an audio cue there is a fader icon to the left of the play icon on the right side. If you click that, a yellow automation line appears over the waveform. The line allows you to draw volume automation on a cue. Pretty useful for make clean slow fades that you can visually see. 

Pitch Shift and Time Shift – In the same window there is a box you can check for Pitch Shift. Checking the box allows you to alter the rate at which the audio is played to manipulate the pitch. If you have the pitch shift box checked, the audio’s pitch will rise with a faster rate, and lower with a slower rate than 1.
At the same time, if Pitch Shift is not checked the rate may be changed to shorten or extend a piece of audio without altering its pitch. That’s time shift at it’s best, and QLab 3 has a very decent algorithm.

Conclusion
Believe me when I say I understand that QLab is expensive. It’s going to be a bit rough purchasing it out of school, but there are great student discounts, so if you’re in school jump on it. Otherwise, you can also rent a license for a short period of time at a fair price.
I have a pro audio license because I’m a sound guy; makes sense. You can spread that license to three machines. The idea is one at home, one on a work system, and a back up system in tandem. That’s pretty generous. Here’s a link to their pricing.
If there’s one piece of wisdom I can pass along today it’s this: learn the in’s and out’s of QLab well, and you will become a much better designer in the theatre.