Selenium IDE is a graphical interface to selenium.
It is a Firefox plugin that does record and playback the interactions with the browser. its is just one of the very popular and useful ways to write selenium scripts.
To use it you need to install the firefox plugin found at-
http://seleniumhq.org/download/ under heading “Selenium IDE”.
After installing IDE you just need to start the recording and interact with your website like click on links, submit form etc. Once you are done just stop the recording and Play it. This will perform all the interactions you have done with the website.
In more professional working model of selenium you can save/export the recordings in PHP, Ruby etc. scripts and integrate it wil a framework like PHPUnit, ruby etc. This will save your lot of time in writing the scripts manually.
In the below article you can find more information on how to integrate the selenium IDE script with PHPUnit.