Client-side refers to operations performed by the client in a client-server relationship. For example, Javascript is a client-side scripting language while PHP is a server-side scripting language. This web page has elements utilizing Javascript as well as PHP. The Javascript elements such as the "Layout Switcher" in the top right corner is written in Javascript and your browser (the client) does all the work to perform the task of switching layout. However, most of the content on this page is controlled by PHP scripts executed by the web server.