The topic of UML is way to big to be dealt with in any meaningful way in a forum such as this. I'd suggest going to http://www.omg.org and starting there, where you can download the specification(s) for free.

For me, one of the most relevant UML packages for 8051 and other MCU-based embedded development efforts is the State Machine package. I have sometimes mentioned state machines (FSMs) and Petri Nets in this forum, but have pretty much settled in with using UML-compliant statecharts to get the best of both FSMs and Petri Nets. I have recently purchased a software tool that will allow me to graphically design hierarchical statecharts, simulate them, perform formal verification on them, and generate C code -- so recently, in fact, that it has not arrived yet. Perhaps a short product report/review might interest a handful of people here in the months to come.