Second, the majority of retail, commercial and governmental web applications have a requirement that calls for the use of “server-side” code. This is code, such as Python, that runs on a server and not in the browser application. The most common functionality of server-side code is passing data back and forth between a database and a web application. For example, if a web app asks for a username and password, that user name and password are almost always stored on a server somewhere and not, for security reasons, in the web page and on the client browser where it could be very easily stolen.
A short note on jQuery.
The absolute minimum recommended reading list: