Selector in android? OR How to give effect on controls like button?

Selector in android is the pressing effect. if user presses the button and u wanna give effect for that , then we use selector in android. Here are the steps for the same 

1. to use selector in android we create xml file in drawable folder 
2. IN above code 
"@drawable/meritlist_hover" : this the image when user presses the button
"@drawable/meritlist"       : this the normal image without press
3. Now use this name to set image in ur button and when user pressed the button it will take image which is specified in selctor.xml file.