There is a lot of documentation on the selenium website about how to use selenium. I tend to use the IDE client to record actions and then port the code for those actions into NUnit so I can run tests automatically on Selenium RC. You can port the code from Selenium IDE into a number of different languages for use on different testing frameworks. You can also manually run commands by directly typing them into the Selenium RC server.
There is a great guide here for learning how to use the IDE: http://seleniumhq.org/docs/03_selenium_ide.html
And a great guide for using the remote control over here: http://seleniumhq.org/docs/05_selenium_rc.html
It looks like they've recently updated the documentation on the Selenium website since last I've looked, so there is now a lot more information available.
Also, as a final note, I've used Selenium IDE to automate repetative tasks while that come up while testing during development. If you have any long forms your always filling out, make yourself a short test case you that fills out the form for you.