When the application is Stable We can go for Automation to perform Regression testing to all the iterative/ Incremental builds.
If we are using Agile process to develop the project then we can't wait untill the project/ application in stable. The recruitments [test cases] Which are stable can be Automated.
What kind of application can be Automated by using Selenium
Selenium Supports all kinds of web Applications
Selenium Supports Multiple languages like Java, C#, PHP, HTML e.t.c to write the scripts.
We can choose any languages to write the scripts
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Selenium components :
Selenium is majorly build on 4 components
1) Selenium IDE 
Integrated Development Environment
2) Selenium Rc Remote control
3) Selenium Web Driver
4) Selenium Grid.
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