Sites that are networks have many sites under their single umbrella. Once you belong to a network, you can usually visit all of their sites for the same price, making it a much lower cost if you visited their sites individually. There are some networks that specialize in one kind of porn or another, such as MILFs, Orientals, BDSM, porn stars, and such. Basically sites in a network, especially a large one, can be defined as sister sites which means that you will be able to enhance your variety of sites as well as the amount of hardcore content you will receive.

Learn more about porn networks in Wikipedia.