Beginer's Guide On Selenium - 1

Introduction to selenium

What does selenium do ? 
You can use opensource(ie free!) - Selenium tool ( selenium IDE is a plugin to firefox) to record and playback tests (like WinRunner, QTP). You can then export the recorded test in most language e.g. html, Java , .net , perl , ruby etc. The exported test can be run in any browser and any platform using "selenium remote control". 

Selenium Flash Demo(Highly recommended for beginners) : 
http://wiki.openqa.org/download/attachments/400/Selenium+IDE.swf?version=1

What are the components of selenium ? 

Selenium IDE - Plugin to Firefox to record and play test in firefox
and also export tests in different languages. The most appealing
format is the html test case which seems to based on fit html.
Selenium RC- Allows playing of exported test in different platform/OS
Selenium Grid - Allows to control lots of selenium machines(you typically dont need this only for load test -and hence may ignore it)..

Other Project Management/wrapper tools around selenium:

How does Selenium remote control internally works ? 
Selenium RC launches the browser with itself as the proxy server (hence you may get certificate warning in some modes) and then it injects javascript - to play the test. This also means it can easily work in ALL browsers/platform - and it can be easily used to test AJAX(unlike professional tools).