If you need a quick reference guide for the UML notation, check one of the following, IMHO, great UML cheatsheets (in no particular order):
- http://refcardz.dzone.com/refcardz/getting-started-uml :Newest reference card, covers all major diagrams
- http://msdn.microsoft.com/en-us/library/dd409437.aspx :Very complete. Includes activity, component, class, sequence and use case diagrams (menu on the left). The only thing that should be improved is that the name of the elements is not shown directly in the digram but explained in tables below
- http://www.digilife.be/quickreferences/QRC/UML%20Reference%20Card.pdf: Very compact. UML 1.0. Covers class and sequence digrams.
- http://www.holub.com/goodies/uml/ :Lots of examples to help you understand the semantics of each notation element. UML 2.0. Covers all major diagrams.
- http://www.scribd.com/doc/399157/UML-20-Cheatsheet :Very well explained. UML 2.0. Covers all major UML diagrams
- http://loufranco.com/blog/assets/cheatsheet.pdf :To the point, example diagrams annotated with the name of the modeling elements used in the example. Class, use case and sequence diagrams covered
- http://edn.embarcadero.com/article/31863 : Very complete, even covering component and deployment diagrams (which is not the case for most of the previous ones). It used to include self-tests to quickly check your knowledge of the notation but this part seems to have been removed from the site.
- UML Diagrams: Detailed overview of all UML diagrams (including the new features of latest UML versions). It’s more a reference guide than a cheatsheet though, since each diagram has its own dedicated page.
Some are quite old but I still like them and the main elements of the UML notation have not changed during all these years (except for the interpretation of the includes and extends relationships in use case diagrams; you should be careful with this).
And if you want to learn more than simple notation details and go deeper in your understanding of your UML language, our selection of UML books may be helpful as well.