With Josh's comment in mind I try to at least narrow down the answer -
Audio bus
This is an auxiliary audio channel (or track). Instead of adjusting each individual channel you have (one for each track) you can send the output of tracks, or create a group that contains selected tracks, and send the mixed output of these to a audio bus. This allow you to set effects and adjustment on the bus once instead of the same settings for each individual channel.
mastering
Is the process of finalizing the mixed down track (output of all) for commercial usage. This is where you increase the output gain, adjust the EQ and so forth. Normally this process is not performed by the mixing engineer or producer, "you" in most cases. Usually a track is sent off to mastering experts as this is its own art on its own.
automation
Automation is a tool to help "record" fader movements, plugin settings so they "move around by themselves" when playing back the tracks. this way you don't have to remember all settings for each playback or have 20 hands to adjust everything at once.
It's kind of programming the settings but with visual aided tools. The data can be stored with the project and when you playback you will know that all settings will be adjusted exactly the same way - every time.
latency
Latency, or delay, is a term used to describe the delay it takes for a sound to get through the audio hardware and to the loudspeakers. The more latency the more delay you will experience. The lower the better. This also affect MIDI as when a note is played it will take some time before the sound produced by the soft-synth gets to the speakers.
The reason for high latency is based on hardware capabilities and drivers. The better hardware and drivers, the lower latency there will be. The goal is always to get as close to 0 as possible.
I can set the MIDI program change number for a track but when is that sent out?
MIDI data is a stream of bytes sent from the DAW to the MIDI port. These are sent as packets containing various data. Some are for the note being played (ie. C#), others is to change programs and so forth. Consider the packets as commands to the synth.
It's sent when the sequencer comes to the point which such data is stored.
What's the difference between adjusting the volume and trimming?
Both are the same. Trimming is perhaps a more detailed process of adjusting, to fine tune if you will. The goal is to avoid clipping/distortion in the final mix. It's normal to start with a low setting such as -15 dB to -10 dB and then "trim" while more tracks are added.
If you work in 32-bit float mode in your DAW these settings affect the quality in a very small degree.
Trimming can also be used with EQ settings to trim of high and low frequencies to allow more room for other frequencies.
How can I increase the volume of a track without distorting the audio?
See point above with mastering.
If mastering isn't an option, try by adding compressor (dynamic range, not the data) and limiter (in principle they are the same, but they have different approaches). Play with pre-settings from the plugins and adjust as you want.
The compressor will make the sound more even while the limiter is the one adding loudness to the whole mix.
An un-mastered track will in many cases sound flat. When you deliver a mixed track for mastering the max volume is often low, in the range -4 to -3 dB. This is normal.
How do audio effects and VST plugins work
Audio produced by the DAW is buffered and delivered to the plugin. The plugin process it based on resolution and frequency and delivers back the buffer to the DAW which then forwards it to the driver for the sound card (ASIO driver in most cases).
Most DAW allow to set EQ before or after it has been processed by the plugin.
See also:
http://www.soundonsound.com/sos/dec00/articles/vst.asp
