C or Ansi C or C++

I have a query and I thought that this forum would be the appropriate one to ask. Here it goes:- What is the difference between ANSI C and C? Let us say that a person wants to take part in coding contests and opts for C (instead of ANSI C) as his programming language ( which obviously isn't a wise act ), then will he incur any difficulties? If a person knows C,then does he needs to learn ANSI C from scratch or is ANSI C just a method to provide a fixed set of rules to make programming in C uniform over the world? Another thing, is their anything as ANSI C++?