TAO - Track-At-Once is used for creating Multisesion data discs in most cases. But can also be used for creating Non-Multisession data discs. It can also be used for creating Audio CD's, but not recommended as your car and home stereo can only read/play the very first session on the Audio CD. Only your computer would be able to read all the sessions of an Audio CD created in TAO mode. TAO mode always reserves space for the next write session, so useses a larger overhead.
DAO - Disc-at-Once, Normally used to create an audio CD, VCD, SVCD, etc. But also used for creating data CDs that you are only going to write too once. It allows more space for the write as it does not reserver space for the next lead-in. And if you are going to create a Master CD for Duplication, you always need to use DAO or SAO. These formats are also used for writing to the R-W subchannels.
SAO - Session-At-Once was created for those Mixed Mode type CD's that needed to be created for Duplication. So you can have one session as Audio, and the next session as Data. And still be able to create a Master CD for duplication.
There's more to all this of course, but this is a very rough overview.
