Validation is testing to determine whether the user entered into the field.
After you determine that something was entered, you can then also check to see whether what was entered is a number or a character and if it is in the correct format.
This takes a lot of resources and makes for a slower-paced form for the user.
Some sites on the Internet don't carry this inputted information back to the form page, and the user is then required to enter all the information into the form a second time.
Obviously, this may cause people to leave your site for another.
If the data is incorrect or not valid, you can send back a response stating this.
If, however, when the user clicks the Submit button, a scripting language that is part of the overall HTML page is initiated to check the validity of the data before it is sent to the server, this is client-side validation.