Client Side: This is code/routines that are not processed at all by the server. Code is written in scripts (javascript usually) - plain text commands that instruct the client to do something. Generally used for performing dynamic effects, such as image rollovers, or displaying message boxes, or even validation of data entered into a form (i.e. email address is in the format of an email address ... contains the @ symbol).
Server Side This is code/routines executed only on the server. Code can be written in scripts as well (vbscript usually), but in the case of ASP.Net this is the compiled lanuages like C# / VB.Net. Used for processing content and returning data.
i.e.
You call up a webpage and a message box appears saying Welcome to "my site". This is client side. It did not require any request from the server to have that message box appear. That code was run on your computer/browser. But the page you requested displays a list of movies playing in your town. That list was populated/retrieved through a call to the Database (located at the City movie repository...or whatever) i.e. the server to retrieve that data for your town.
Hope that helps.
:cool: