client side validation is performed in the clients browser using javascript. It can be used to trigger other events such as conditional validation, or to enable or disable another form element. it can also be used to reformat the data is sent.
The drawback to client side validation is that it can be avoided by simply turning off javascript in the browser.
Server side validation is performed through PHP code on the server. This is a much more secure option since there is no way the validation can be avoided.
Server Side validation cannot be used for conditional validation, or for enabling or disabling form elements though.
I always recommend to use Server Validation since it is more secure. If you need the extra functionality offered by client side validation, than you can add that as well, but dont rely on it.