The definition of a tarot deck varies quite a bit from person to person. The bare bones definition is a deck of 78 cards: 22 majors and 56 minors (of four suits, ace - 10, and four courts per suit).
There have been, historically, deviations. An example is a Minchiate deck, which has 40 majors, plus the Fool. Modern deck designers have played with this structure as well, sometimes adding a few cards to the majors or adding a court to the minors, or even an entire fifth suit. Some would argue these are not tarot decks.
In addition, there are decks that follow what I call the bare bones structure but deviate wildly from the familiar Golden Dawn (Thoth and Rider Waite) images and