These are enhancements to your HTTP server that allow programmers to write the code that makes your website talk to databases, handle e-commerce orders, etc. The most popular scripting languages are PHP and ASP.Net.
Keep in mind that if you're used to viewing Facebook through the secure connection (https), you'll need to revert to the regular connection (http) in order to add Twitter to your Facebook page.
To be sure the site is secure, look for a small locked padlock icon (often located in the right corner of the browser window) or check for "https" instead of "http" in the URL.
When you visit a secure site, your browser should show an image of a locked padlock, and instead of the usual http, the site's address will start with https.
These variables can provide your script with real information about the web server, your user's session variables, HTTP cookies and more.
In order to display web pages, the server must be configured with an HTTP server program that responds to requests and shows pages.
If you decide to add support for one of these scripting languages, you will need to configure them to work with your HTTP server.
If you want to know whether your browser is on a secure website, the beginning of the web address will be https instead of http.
Look in the URL line and make sure the credit card enter page has an HTTPS and not just an HTTP to ensure your safety.
Here's a description of them at the Wikipedia (http://en.wikipedia.org/wiki/Help:Footnotes)