Probably one of the most underrated games of all time.
When you start the game you only have access to the easiest stages. By winning the races you earn balloons which after collecting so many you gain access to more stages. Each "World" five racing stages, a boss race and a challenge (usually a battle stage of some kind). Once you complete the boss race you will have to beat the races in the silver coin challenge. In addition to just playing through the races there is a time trail mode.
Once you beat through the game completely you'll unlock the mirrored courses. It might sounds like it is just the same game over again, but the difficulty is cranked up a bit. Also, you'll have to retrain yourself on the stages.
One of my favorite features is the multiplayer though. You can play any course you've unlocked with any vehicle that can get through the level (some water stages are hover only). It makes for some very fun multiplay action as each play could be using a completely different vehicle.
Overall this is a very deep game with more than enough stuff to do. The story is the weakest point, but considering you wont largely care about the story, there is little bad about this game. I can say that if you own an N64 do yourself a favor and play this game!