Server side code get executed on the web server in the response of request for any aspx page where as client-side code get executed on the client browser (e.g validation of controls, dynamically hidding and displaying some of controls or there values etc)
For implementing validation in ASP.Net MVC, have a look at below article:
http://weblogs.asp.net/scottgu/archive/2010/01/15/asp-net-mvc-2-model-validation.aspx[^]
Creating Client and Server-Side Form Validation using the Validator Toolkit for ASP.NET MVC[^]
http://nickstips.wordpress.com/2011/08/18/asp-net-mvc-displaying-client-and-server-side-validation-using-qtip-tooltips/[^]