Any code that runs an instruction on the server-side (usually a web server) and sends data back to the client (usually a web browser). They are especially useful when building sites that have dynamic content, because you are able to write scripts which pull information out of a database, rather than building the HTML manually. They also don't rely on client-side techniques like JavaScript or DHTML to be dynamic because all the dynamic content is put into a standard HTML page on the server side.

Some common server-side languages are Perl, Microsoft's ASP and Sun's server-side Java.