Fungible — being something (such as money or a commodity) of such a nature that one part or quantity may be replaced by another equal part or quantity in paying a debt or settling an account.
In a nutshell, a fungible token has a total supply of X and the same price for all tokens, but a Non Fungible Token has a total supply X and it can have a different price for every token, each token is special and has a different appreciation.
What does ERC mean?
ERC stands for Ethereum Request for Comments. An ERC is authored by Ethereum community developers in the form of a memorandum describing methods, behaviors, research, or innovations applicable to the working of the Ethereum ecosystem. It is submitted either for peer review or simply to convey new concepts or information. After core developers and community approval, the proposal becomes a standard.
It is the most common and well-known standard within all crypto community. 99% (if not all) issued ICO tokens on top of the Ethereum implements this standard. Actually, it is just a simple set of functions that your token code has to have.