Going Twice [I have posted something similar to this before but got no response so i allowed it to slide]
Though the other one - i did not expantiate - i will do that now.

[Client-Sided Validation]
This area deals with how to make sure the major fields in your form are filled before submission.
For instance if you are talking of a login form. It is important to make sure the username and password are filled.
In a registration form, maybe we are expecting area codes or 3 characters, we need to make sure that 3 numbers
of entered not 3 alphabets and not just any character like lol.

So we are talking about putting the right data types expected, not leaving blanks, and this also covers makin a textarea have a maximum length,

These stuffs are done in client-sided validation without reloading the page (and without wasting client's time)

[Server-Sided Validation]
Now you have submitted the necessary details, but u need to crosscheck some things with say a database, this is where
you now crosscheck the entered information to see if they are correct.
For instance the username/password must be checked against the records in the database,

This will usually reload the browser into the same page or different page.

[Ajax Validation]
This will combine both client and server-sided without leaving the page.

Going, going, gone!
After reviewing so many sites, i discover that form validation seem to be a major cause of problem.
If you are interested in how to validate forms properly, please raise up your hand.

:-x

Going thrice! This time there is no ban for even moderators