As an example, you might have a form like this:
The point is, that regardless of what you do on the client you should validate the inputs on the server. On a public form you have little or no control over what is happening on the client. All you can really control is what goes into you application or database.
Cleverly, Coldfusion looks for the "_required" or "_date" (or _integer or _float etc. - see the docs) that matches the name of a form element and then validates that element on the server - displaying the message you have chosen to the user and asking them to go back and fix the problem. Yes, this is server side validation because the server is doing the checking, but is it secure from malicious attack? Nope. All a nere-do-well has to do is remove the "myDateOfBirth_date" form element from his custom form or CFHTTP call. The server's validation is based on these inputs being present. If those form elements are not part of the request, the server will not check them.
Server side validation is the process of painstakingly checking the inputs and making sure they contain what they are supposed to contain. Most of my server side validation code looks like this:
In my opinion, if security and data integrity are your primary concerns, you should start with server side validation and put it above client side validation in importance. You should add client side validation to reduce round trips to the server and maximize the user experience but don't rely on it to keep your site safe or your data sound.