Unabridged Audio Book - An audio book described as unabridged is a complete cover to cover reading/performance of the whole book.
Abridged Audio Book - An abridged audio book is a reading of a book with passages removed and with the focus more on the "action".
CD - The most popular and most expensive. An unabridged book can run to 25 CDs! If you listen in the car & have a CD auto changer, this is a convenient format.
Cassette - Cassettes are durable & (I find them better for the car - music in the autochanger & a book in the cassette player). They are also usually about 25% cheaper than CDs and therefore represent excellent value for money.
MP3 - The latest format. MP3 audio books are now available, but they require (obviously) an MP3 player or computer to play them. They will NOT play on a standard CD player.