"RFC" stands for Request for Comments, and this name (used since 1969, before the IETF existed) expresses something important: the Internet is a constantly changing technical system, and any document that we write today may need to be updated tomorrow. One way to look at the IETF is as the group of people who work together to improve the technology of the Internet on a daily basis.
As well as producing RFCs, the IETF is a forum where network operators, hardware and software implementers, and researchers talk to each other to ensure that future protocols, standards and products will be even better. This is not the only technical forum for the Internet, of course. But it is the forum where the basic technical standards for Internet protocols are set and maintained.
The IETF does not standardize transmission hardware (we leave that to organizations like the IEEE and the ITU) and does not standardize specialized application layer protocols. For example, we leave HTML and XML standards to the World-Wide Web Consortium. But the IETF does standardize all the protocol layers in between, from IP itself up to general applications like email and HTTP.