Javascript is code that is written as text within the HTML source of the web page and is often used to check the data entered into a webpage form and then the visitor's web browser interprets and executes the code rather than a web server.

Check it out, some free javascripts with easy installation instructions and lots of cool stuff, there's even a tutorial on how to write it; http://java-scripts.net/

PHP is a powerful scripting language adopted in commercial websites. PHP (an alternative to Perl and the cgi-bin) is easy even for non-coders and can be easily integrated into HTML pages (just like Javascript). More info: www.php.net

Perl (Practical Extraction and Report Language):
The most common scripting language for processing Form data.
See: How to embed your HTML email with tracker code?

CGI (Common Gateway Interface): A protocol for gathering data from a Form and sending it to a program or Script such as Perl (a transfer interface) method for processing a Form. CGI scripts are use when you want 2-way communication with your readers.  
For a simple quick non-technical layperson over-view about CGI

CGI scripts are often written in the following languages:

Perl, C/C++, Visual Basic
Scripts written in the above languages are called server-side scripts as they are located on the server and are executed only when a user clicks 'submit' on a fill-out form.

And JavaScript, VBScript - are known as client-side scripting languages and are located in HTML documents and are executed by your browser.

Matt's Script Archive for free CGI Scripts that you can install on your web sites with detailed step-by-step instructions www.scriptarchive.com/

Jcqueline D. Hamilton's on-line tutorial "CGI Programming 101." teaches you the basics of writing Perl CGI Scripts, step-by-step www.cgi101.com/class/