Server side code will process at server side & it will send the result to client. Client side code (javascript) will execute only at client side.

server side code make a round trip two server and post back happens. client side code runs in the browser itself and post back won't occur

Server side code means code writing in aspx.cs(C#,vb.net etc) page where as client side coding means writing code on aspx(javascript etc)

1) Control to validate

2)Validation Expression

3)Validation group