An Audio track in Pro Tools LE is used for recording audio to, such as a guitar track, vocal track or bass track. Most Pro Tools sessions consist primarily of Audio tracks, as this is the only type of track in Pro Tools that you can record audio to.
An Aux track is a track that lets audio pass through it. Aux tracks are typically used to route tracks to a single output, such as combing all of your guitar, vocal or drum tracks. Aux tracks are also commonly used for effects sends, such as a reverb or delay channel for a vocal track. Additionally, you can use Aux tracks to monitor external devices or virtual instruments.
MIDI tracks are used in Pro Tools LE to control virtual instruments, such as a drum machine, synthesizer or keyboard. MIDI tracks do not contain any audio and instead contain data, such as what note to play, how long to play it and how loud to play it. To use a MIDI track to create sound, you must route it to a virtual instrument, usually hosted by an Aux track.
Instrument tracks are combinations of MIDI tracks and Aux tracks. They combine the MIDI recording capability of a MIDI track with the virtual instrument playback capability of an Aux track, eliminating the need to route a MIDI track to an Aux track to hear the virtual instrument it is controlling.
Master Fader tracks are used to monitor the outputs of your Pro Tools LE session. Typically, all of your Audio tracks, Aux tracks, MIDI tracks and Instrument tracks will be routed o the output Analog 1-2. By creating a Master Fader, you can monitor the output of Analog 1-2 and make sure that its output is not overloaded by the sum of tracks being sent through it.