Client-side means that the action takes place on the user’s (the client’s) computer. Server-side means that the action takes place on a web server.
So what do we mean by action? And why does some action take place on your computer and some on a server?
When we say action we mean all that work that programming languages do. You will remember from our term about programming languages that they are the doers of the web.
Read the whole article here: http://skillcrush.com/2012/07/30/client-side-vs-server-side/