I was just discussing this with some friends recently.
I tried to explain that tarot is an oracle but not all oracles are tarot. Tarot is an oracle with uses a defined set of cards while an oracle can be nearly any random set of images such as tossed coins, tea leaves, flocks of birds, patterns of stars in the sky, movement of clounds, or random words picked from a book.
Tarot is a deck of cards that can number 22 or 78 (with a few minor exceptions) There are two defined subsets of Trumps and minor pip cards. There are a few other conventions that should be followed like naming of major trump cards and court cards number but these are traditional and some have changed them with a mixed level of success.