The SDLC is a general notion of the phases required for developing software:
vision > requirements > architecture > coding >
testing.
RUP is a product of Rational Corporation (recently IBM) and is a much more detailed process "guide" so to speak. RUP is basically a set of artifacts (with templates) that one should pick a suitable mix for a particular project. Also, RUP is meant to be an iterative process -- something which the SDLC doesn't talk about at all.