Informational: Intended to provide information for the Internet community. It does not state an IETF standard of any type, although other organizations' standards are sometimes republished as Informational RFCs for convenience.
Experimental: A specification for use within a limited-scope experiment.
Best Current Practice (BCP): An IETF recommendation as to the best practice in some area or the best practice that some part of the IETF complex should follow.
Proposed Standard: The first step on the standardization ladder. It indicates that the IETF is likely to standardize something in the particular area and it should not have any obvious flaws in it. Not considered to have any assurance of stability.
Draft Standard: The second step on the standardization ladder. It indicates a reasonably stable specification. There must be at least two independent interoperable implementations of any standard to reach this level. An emphasis on interoperability is a hallmark of the IETF process.
Internet Standard: The third and highest step on the standardization ladder. It indicates substantial successful deployment and widespread interoperability.
Historic: The graveyard state for documents that are no longer valid.
In addition, RFCs contain information at the top of their title page indicating whether they modify or make obsolete an earlier RFC.