Ardour offers three track types depending on the type of data they contain, and differentiates between three track modes, depending on their recording behaviour.

Track types

An Ardour track can be of type audio or MIDI, depending on the data that the track will primarily record and play back. However, either type of track can pass either type of data. Hence, for example, one might have a MIDI track that contains an instrument plugin; such a track would record and play back MIDI data from disk but would produce audio, since the instrument plugin would turn MIDI data into audio data.

Nevertheless, when adding tracks to a session, you typically have an idea of what you need to use the new tracks for, and Ardour offers you three choices:

Audio

An Audio Track is created with a user-specified number of inputs. The number of outputs is defined by the master bus channel count (for details see Channel Configuration below). This is the type of track to use when planning to work with existing or newly recorded audio.

MIDI

A MIDI track is created with a single MIDI input, and a single MIDI output. This is the type of track to use when planning to record and play back MIDI. There are several methods to enable playback of a MIDI track: add an instrument plugin to the track, connect the track to a software synthesizer, or connect it to external MIDI hardware.

If you add an instrument plugin, the MIDI track outputs audio instead of MIDI data.

Audio/MIDI

There are a few notable plugins that can usefully accept both Audio and MIDI data (Reaktor is one, and various "auto-tune" like plugins are another). It can be tricky to configure this type of track manually, so Ardour allows you to select this type specifically for use with such plugins. It is not generally the right choice when working normal MIDI tracks, and a dialog will warn you of this.

Track Modes

Audio tracks in Ardour have a mode which affects how they behave when recording:

Normal

Tracks in normal mode will record non-destructively — new data is written to new files, and when overdubbing, new regions will be layered on top of existing ones. This is the recommended mode for most workflows.

Non-Layered

Tracks using non-layered mode will record non-destructively — new data is written to new files, but when overdubbing, the existing regions are trimmed so that there are no overlaps. This does not affect the previously recorded audio data, and trimmed regions can be expanded again at will. Non-layered mode can be very useful for spoken word material, especially in combination with push/pull trimming.

Tape

Tape-mode tracks do destructive recording: all data is recorded to a single file and if you overdub a section of existing data, the existing data is destroyed irrevocably — there is no undo. Fixed crossfades are added at every punch in and out point. This mode can be useful for certain kinds of re-recording workflows, but it not suggested for normal use.

The screenshot on the right shows the subtle difference between an overdub in normal mode (upper track) and one in non-layered mode (lower track). Both tracks were created using identical audio data.
The upper track shows a new region which has been layered on top of the the existing (longer) region. You can see this if you look carefully at the region name strips.
The lower track has split the existing region in two, trimmed each new region to create space for the new overdub, and inserted the overdub region in between.

Channel Configuration

Ardour tracks can have any number of inputs and any number of outputs, and the number of either can be changed at any time (subject to restrictions caused by any plugins in a track). However it is useful to not have to configure this sort of thing for the most common cases, and so the Add Tracks dialog allows you to select "Mono", "Stereo" and few other typical multichannel presets
The name of the preset describes the number of input channels of the track or bus.

If you have configured Ardour to automatically connect new tracks and busses for you, the number of outputs will be determined by the number of inputs of the master bus, to which the track outputs will be connected.
For example, if you have a two-channel master bus, then a Mono track has one input and two outputs; a Stereo track has two inputs and two outputs.

If you you set Edit > Preferences > Audio > Connection of Tracks and Busses to manual, then tracks will be left disconnected by default and there will be as many outputs as there are inputs. It is up to you to connect them as you wish. This is not a particularly useful way to work unless you are doing something fairly unusual with signal routing and processing. It is almost always preferable to leave Ardour to make connections automatically, even if you later change some of them manually.