It's a protocol which basically sets out how the computers talk to each other. Think about it as the computers' equivelant of a language. We understand each other because we both speak English, grammar, syntax, etc... are the same. Our computers can understand each other because they both use TCP/IP. Error checking, etc... is determined by the protocal, as is what info is contained in each frame.
I won't get into too much detail here, as it's all available easily on the internet, with better explanations than I can provide. I'd start with google for more info.