The "cgi-bin" directory you often see in URLs refers to CGI (Common Gateway Interface). In basic terms, the cgi-bin directory contains programs that make things happen behind the scenes on a website, mostly related to coupling user input via a web browser to another application running on the web server. This can be simple stuff like compiling the information entered into a web page form and sending it as an email, to complex things like managing commerce sites (customer and stock databases, etc.), or running a wiki-based site. CGI is not the only way of making websites with dynamic content, but it is popular and very widespread.