A tarot deck is a classic set of 78 cards that fall under the category of major or minor arcana. From there, the cards are divided into four suits (Wands, Cups, Swords, and Pentacles). Tarot cards are studied and interpreted by expert readers, primarily to speak to a person's life story.
Oracle cards are more free-wheeling from the set structure of tarot. An oracle deck can essentially be whatever the creator of the deck wants it to be — they pick how many cards are in it, what sorts of imagery it'll use, and what purpose it's supposed to serve. There are many different types of oracle decks that come in all shapes, sizes, and functions. The basic set-up is that they have images and words, along with a book that explains ways in which a reader should interpret them.