Pleas:
Guilty - You admit that the act is prohibited by law, that you committed the act charged, and that you have no defense or excuse for your act.
Nolo contendere / no contest - You do not contest the state's charge against you. While it is not an admission by you that you are guilty, it will result in a conviction on your record.
Not guilty - You are informing the court that you deny guilt or that you have a good defense, and that the state must prove what is charged against you.