Simple robots extend Robot. They don't take damage when they hit walls, and can't use non-BlockingCall?s, CustomEvents or access files. They can use FiringAssistance (due to a bug, so can AdvancedRobots).
More complex robots extend AdvancedRobot.
Yes, but they are few. There is an ExtendsRobot competition somewhere on this wiki, kawigi.robot.Girl, for example, also competes in the roborumble and meleerumble. I don't recommend starting out learning Robocode by trying to make a competitive Robot. -- Martin
Typically the source of such a question is competitions that don't allow AdvancedRobots (usually with the purpose of making it less likely that people will steal code from existing bots on the Repository). -- Kawigi
Ah, so you are suggesting that the anonymous poster is trying to cheat in a class competition. Lame. -- Martin
I'm not saying that's necessarily the case, just that it's quite possible :-) Also, I have been contacted before by a person asking if they could use some part of Girl in that kind of competition (although it was someone that I knew from another programming community). Girl is good partly because it is a robot designed without the limitations of an ExtendsRobot in consideration, and then adjusting things to allow for those limitations. -- Kawigi