Oh, sorry, I thought that "what is it for" was actually what you wanted to know, rather than "what is it".

How about this, then:

It's a modelling language that defines syntaxes for different types of diagrams relating to system design (eg FSMs, message sequence charts, user interactions, object hierarchies) and a framework for fitting them together to provide a coherent (hopefully :-)) system architecture that can (again, hopefully :-)) be understood by techies and non-techies alike.

It has a string of other claims associated with it, including things like self-documenting systems (if you've written the UML model and developed the code from it, there's no need for further documentation) and making development faster and more reliable with less need for skilled programmers (if your system architect has designed the model, she doesn't need to know the intricacies of C or C++ or Java syntax to implement it - just press a button and bing, there it is).

As I tried to imply above, I'm not really sure that it does actually achieve any of these otherwise laudable aims.