It’s a pretty simple question if you know how the web works, but if you don’t, it is very complicated. To succeed working in the web world, you’ve got to have a firm grasp on the difference between server-side and client-side interactions. If you want the quick answer, here you go:
Server-Side: Server-side interactions require sending data to a server and the server sending data back to you, the client.
Client-Side: Client-side interactions don’t need a server, and data is exchanged or altered on your own computer browser without sending any information to a server.
Now if you need the longer explanation, you’ll first need a little background on how the internet works. The internet is just a mapping of connections between millions of computers around the world. When you type in a web address–such as http://www.google.com/–your computer knows that means go find the computer somewhere in the world that has the website for Google on it (I’m really simplifying things here if you can’t tell). The computer that has Google on it is called a "server". Servers aren’t typical computers. They don’t necessarily have any screens to view anything on, and for sites like Google, thousands of servers are collectively used to house the website.
So now that you understand the web (at least in a basic sense), we can start to learn about what server-side and client-side means with a more indepth explanation. So say you are on Google’s homepage. As a user (consumer) of Google, you can interact with their homepage in a multitude of ways. To understand each type of interaction, I’ll give an example using the Google homepage.
Server-Side: Now, after you’ve typed a search phrase into the search bar, when you click the "Search Google" button or click enter, you’ve now triggered a server-side interaction. Your computer doesn’t have the information it needs to conduct the search you are requesting, so it must send your search across a series of connections to a Google server somewhere in the world, that then sends back the results that it found for you. This is the basic concept behind server-side interactions. Typical server-side scripting languages include Perl, PHP, Java, and ASP.NET (I could keep going, but who is it really helping?). The key to identifying a server-side interaction is noting when your browser page reloads. Server-side interactions require reloading the browser window to show the results that were found. There is one exception to this rule.
Now hopefully you’ve got a firm understanding of the difference between server and client side interactions and languages. If you’ve got any questions, don’t hesitate to ask.