Joomla! Documentation™

Joomla has client-side and server-side form validation capabilities.

Client-side validation

.. is done via javascript while the user is filling in the form fields. It uses the HTML classes required and validate-[xxx] (with [xxx] being a joomla or custom rule; e.g. validate-numeric)

More here: Client-side form validation, Joomla! client side form validation/1.7

Server-side validation

.. is done after having submitted the form and will normally return to the form when not validated with some extra messages. It used the HTML attributes required ("true" or "required") and validate (with value being a joomla or custom rule; e.g. validate="email") This is the minimum requirement, since everything before that can be overridden on the user side.

More here: Server-side form validation