basics of UML

Hi,

I am an IT manager with a poor knowledge of object orientation and UML. I’m looking for simple documents of OO and UML, which are able to teach me the essence of those two abstract things (OO and UML). There are tons of literature, but who can help me with some plain and simple documents that explain the essence of OO and UML ? So, not a book of 200 pages, but a useful URL or white paper.

Thanks in advance.

Kris