iostreams are the formatting part of a two-part (input or output) system. The other part of the system is made up of streambuf streams, which work with input or output of unformatted streams of characters.
You usually use streambuf streams through iostreams, so you don’t have to be familiar with them in detail. You can use streambuf streams directly if you choose to, for example, if you need to improve efficiency or to get around the error handling or formatting built into iostreams.