There are 3 bike categories: Rookie, Team, and Pro Rider. 

Rookie bikes are the entry level tier of bike. When you first start the game, your account will start with enough Coins to get your first Rookie bike. Team bikes are one tier up from Rookie. These bikes start out with better stats than Rookie bikes, plus they can be upgraded further than Rookie bikes. Plus, when you buy a Team bike, you'll automatically get the Team's sponsorship which pays better than any sponsorship you can get from an Rookie bike. When you're riding on a Team bike, it's as if you are the third member of the corresponding team. Plus, two upgradeable team staff will automatically be unlocked and activated when you buy a Team bike.

Buying a Pro Rider gives you the highest performing bike tier in the game. When you buy one of these, you are actually buying the ability to race as one of the actual Pro Riders in the MotoGP sport. So even though the bike looks similar to the Team bike, because you're racing as Rossi or Marquez, your bike will be better than a Team bike and can be upgraded even further than a regular Team bike. Plus, the sponsorship you get as a Pro Rider will pay even better than a Team bike, and all four upgradeable team staff will be automatically unlocked and activated. Most importantly, the only way to participate in the Fan World Championship is to buy a Pro Rider.