Session: A recorded segment of a compact disc which may contain one or more tracks of any type (data or audio). In data recording, there is usually only one track per session, but there may be multiple sessions on a disc. In audio recording, all audio tracks should be in a single session. A lead-in and lead-out are recorded for every session on a disc.
Track: Every time you write to CD, you will create at least one track, which is preceded by a gap. Any session may contain one or more tracks. Multiple tracks within a session may be of the same or of different types â€” for example, a mixed-mode disc contains data and audio tracks â€” though it is more common for multiple tracks within a single session to be all audio tracks.
DAO - writes everything at once, including the subchannels. You can, for example, wite up to 250 MP3s or 700MB of 'em in this way. This mode is used when writing a complete disc in a single pass without pausing. Data cannot be added later, even if the full capacity of the blank disc has not been used.
TAO - up to 99 tracks can be written. One track can have, for example, more than one MP3. This mode is used when writing data to a disc one track at a time. More tracks can be added later if there is enough space left on the disc. This is why Track-at-Once mode is sometimes referred to as Multisession. Only the first session of a multisession disc can be played back on a conventional audio CD player. Single session discs can be played back completely.
SAO - fit as many sessions, of as many tracks, as you like up to the limit of the disc.
The lead-in and lead-out areas signify the beginning and end points of a session. They are not part of the data but contain information about the session itself. Basically, a session consists of a lead-in, data, and a lead-out.