The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. The Transport Layer describes the quality and nature of the data delivery. The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. The Presentation Layer describes the syntax of data being transferred. The Application Layer describes how real work actually gets done.
Most newer designs use the seven-layer model.
The OSI Basic Reference Model has enjoyed a far greater acceptance than the OSI protocols themselves. There are several reasons for this. OSI's committee-based design process bred overgrown, unimaginative protocols that nobody ever accused of efficiency. Perhaps most importantly, X.25 data networks never caught people's imagination like the Internet, which, with a strong history of free, downloadable protocol specifications, has been loath to embrace yet another networking scheme where you have to pay to figure how things work. OSI's biggest problem is that doesn't really offer anything new.