Selenium IDE is an automated testing tool mainly used for testing web applications. I would, rather say, only web applications. It's add-on. It can added in Mozilla Firefox browser. It is very simple to understand and best way of learning is doing hands-on on it. As said before, it is automated testing tool, automated scripts will come into picture. Main strength of this tool is it support javascript. So, the person knowing basics of java or javascript can easily master on this tool. If a person (or tester) doesn't knows about jvascript, then he/she can go through basics of javascript on w3schools.com.

       Advantages of using selenium is it saves a lot of time, easy to understand and use. To use, all you need to do is give URL of your web application in URL textbox provided in UI of selenium. It also records which button is getting clicked, in which textbox which value is getting entered. It records name of button, name of textbox or radio button (the name which was used during cooding phase of web application). If there are huge numbers of test cases that are gtting covered in your one script, then you can load the script and run and simultaneously, you can do your other work. Simultaneous work will not affect its (selenium) work. Write a script and simply execute. It will test the application and will provide the results and will display, if application is failing somewhere. For it's forums, visit openQA.org and you will get every answer for your every question. For downloading of selenium IDE, visit seleniumhq.org.