I'm going to start a tutorial thread where I post short tutorials based on what I know about mixing. Be warned that this is partly my opinions based on what I have learned in school and on my own. Also, if you have a tutorial of your own or if you would like to add anything, feel free to post!

First of all, I am going to cover the topic of busses/parallel sends and submasters.

When you create a bus/parallel, you are basically splitting your tracks into two parallel parts: a wet and a dry part. The original tracks become the dry while the bus becomes the wet. For this reason, when you put an effect on a bus, such as reverb, you generally want it to be 100% wet. If you are sending multiple tracks to the same bus, you can usually control how how much you are sending, but how you do this depends on the DAW. A bus is therefore really useful for certain effects such as delay and reverb, where you usually want a mix of wet and dry. Also, some of these effects can be fairly processing or memory intensive and when you use bussing for them instead of putting the effect on each individual track it conserves processing power/memory.

On the other hand, when you create a submaster, you are sending multiple tracks directly to the submaster track without splitting them into two parts as you would with a bus (i.e. the output for the track will be the submaster rather than the master). This is great for grouping tracks which have a similar function within the mix (such as all of your percussion tracks, all of your bass tracks, etc.), and then putting effects such as EQ and compression on them and/or automating them. It makes large numbers of tracks much more manageable.

I'm sure you know some of the basics of reverb (reverberation). Any space in real life has a specific and unique reverb which consists of the amount of time the sound reverberates and other elements such as how the sounds interact and amplify/cancel each other when they bounce, or how long the sound takes to reach the audience within the space. For example, you may be in an old church in which it takes 5 seconds for the echoes to fade away (decay). It also takes 800 milliseconds to reach the middle of the audience (pre-delay). There are some "dead" spots in which a listener would not be able to hear the reverberation very well, or some spots in which the listener would be able to hear only the reverb, and none of the dry sound of the performers. The shape of the church also tends to cause notes of a lower frequency to be amplified more than those at a higher frequency (EQ).

Basically, when you add virtual reverb to a track in a mix, you are trying to make your music more realistic by putting it in a virtual space such as this church. In terms of mixing, this is called creating "depth" (more on this later when I get to the three essential dimensions of a good mix - height, width, and depth). There are basically two ways to create virtual reverb through plug-ins. The first way uses a plug-in that lets the user artificially create a space by changing different settings (such as decay time and pre-delay time; often there is some form of EQ included too, but often it is best to add a separate EQ. More on EQ later). The second way uses what is called "convolution reverb", which, instead of creating a new virtual space, actually maps out the reverb of a real space using "impulse response". In this way, you can basically recreate the reverb of any space. This is by far my favorite of the two methods. Logic has a great plug-in called "SpaceDesigner" that uses this method, and there is a great free one called "SIR" that I am currently using that I think Nyu001 posted on the free tools thread.

Delay is another way to add depth to a mix, but it tends to be much more artificial-sounding than reverb as it only creates an echo (which can be a good thing if that is the effect you are going for). It is much simpler and basically consists of deciding how much time you want between echoes and how long you want it to take for the echoes to decay. Some delay plug-ins also add a panning element to give the effect more complexity.