Reverb is a natural element of sound when produced inside an enclosed space. Walls, ceilings, and floors all reflect sound. These reflections eventually reach your ear. But since the path taken by reflected sound is longer than the path taken by direct sound, there is a time delay between when you hear direct and reflected sounds. Individual, distinguishable reflections are called echos. When reflections become so numerous that they become indistinguishable, they're referred to as reverb.
Each room produces its own unique reverb pattern. Variables include dimensions and shape of the room (large room = long reverb, small room = short reverb), amount and quality of reflective surfaces (marble reflects more than wood which reflects more than carpet), and amount and quality of absorptive surfaces (a bare room = stronger and longer reverb, a room filled with overstuffed furniture = weaker and shorter reverb). Often, natural reverb can be recorded with the source during the recording phase. In this case, artificial reverb may not be necessary. But often engineers rely on artifical reverb (via outboard hardware and/or software plugins) to help create a perceived space for a track.
If you've followed our advice and taken good notes during earlier phases, you have a room setting in mind as part of your master plan. Quiet, intimate songs are often more effective when placed in a small room setting with short reverb and strong early reflections (both you and the source are close to walls and ceiling). Raucous rock songs are often more realistically placed in a large concert hall setting with long verb and few early reflections (you're a long way from the walls and ceiling, right?). Whatever room setting you choose should help you determine the type of reverb to add.
Here are some other basics you may find helpful. And as always, there are resources at the bottom of the page to help in further study.
Make it real - Remember that the listener's brain is used to hearing sounds within an actual physical space and with natural reverb. If you break these natural laws, your tracks will sound unnatural. That's not always a bad thing, but you should keep it in mind. Select reverb that seems appropriate to the song. A soft vocal in a large hall will lose its intimacy.
AUX work great - Try setting up 2 different reverbs in 2 different AUX slots. When using AUX slots, remember to mute the Dry signal within the verb (you will mix the Wet AUX signal back into the Dry channel signal later). Select one verb to fill out instruments, the other for vocals. Using slightly different verb settings tends to pull the vocals together but slightly separates them from instruments. The approach leads to a less-than-natural mix but works well to help keep vocals out front in the mix (meaning they're easily heard).
Keep it clean - Keep much of the signal dry. There are 2 basic approaches to adding verb to a signal; you can add a touch of verb to the entire signal (via the channel insert), or you can add lots of reverb to a small part of the signal (via an AUX channel). Whichever approach you choose, keep in mind that any reverb you apply will tend to soften the sound. Too much (or the wrong type of) reverb can make a lead vocal unintelligible. To avoid this, you might add a slight delay (try 80ms or so) to the verb. This helps retain clarity by causing the listener to hear the perfectly dry vocal just before the verb smoothes it out. Don't overdo reverb. To determine how much is needed, try soloing the track and adding just enough reverb so you can barely hear it. Write down the setting. Then, unsolo the track (listen to all tracks) and repeat the process. Write down the setting. You may find success in picking a setting halfway between the two. But, as always; trust your ears.
Push 'em back - Use reverb to push instruments/vocals to the background. Remember that the closer you are to a sound source, the lower the wet/dry ratio is. So tracks with little verb (little wet, more dry) will sound closer and more clear. You can use this to your advantage by adding slightly more reverb on background vocals than on lead vocals. Do the same with rhythm (as opposed to lead) guitar. This tends to blend the backing tracks together while forcing the lead tracks to the foreground.
Bass-icks - Generally, adding reverb to bass instruments (kick and bass guitar, for example) tends to muddy the mix and make the bass sound... well... icky. Often, reverb plug-ins have high-pass filters built into them. Use these (or an EQ plug) to roll off lows before adding reverb.