Client Side Validation
Client-side validation is quick and responsive for the end user.
client-side validation is the more insecure form of validation. When a page is generated in
an end user’s browser, this end user can look at the code of the page quite easily (simply by right-clicking his mouse in the browser and selecting View Code).
Server Side Validation
When validation occurs on server, where application resides it is called server side validation.
The more secure form of validation is server-side validation.
It is more secure because these checks cannot be easily bypassed.
The best approach is always to perform client-side validation first and then, after the form passes and is posted to the server, to perform the validation checks again using server-side validation.