Server Side vs Client Side languages are terms used in website development languages, according to execution website development languages are divided into two categories, these are server side and client side languages.

Servier side Languages

Those languages which is executed on server and converted to HTML, these languages are also called as Back-end Languages. Most common server side languages are

• PHP
• ASP.net
• Ruby
• ActiveVFP
• Python

Client side Languages

Those languages which are executed at client side (browser) are called Client Side Languages or Front-end Languages, Most common Client side languages are

• HTML
• CSS
• JavaScript
• JQuery
• VB Script
• Ajax

In Server Side and Client Side terminology web development Languages are categorized on the basis of its execution place, either a language is/will execute on server or User/Visitor Browser.

JavaScript is Client side language which can also be used as Server side language, it means JavaScript can be used to execute on server to produce HTML and JavaScript. Currently Node.js is best example of it.

Example
When we type www.google.com in browser it sends request to Google Server, On Google Server website is written using both client and Server Side Languages. Server side languages are executed on Google Server which returns HTML which is client side language, this HTML and other Client Side Languages like JavaScript and CSS are sent to our Browser, which are all Client Side Languages and our Browser can understand and execute it to produce visual look for us.

Why We Need Both Server and Client Side Languages

Both plays important role on its places, Client side languages are used for Static Data, Visual Design, Styling, Animations, Validations etc. as these tasks are done on browser so they are faster.
If we somehow do some of these tasks by client side languages like we do validation by server side it will not be as faster as Client Side Languages do.

Where as Server Side Languages are used to Connect to database, fetch data from database, save or change data in database, our business logic. As database holds data of all users and we do not want a user to see data of another user, and we do not want users to see and understand our business logic. That’s why we write this portion of website in some Server Side Language.
we can also connect to database via Client Side Languages but in that case users will be able to see our database connectivity and they can alter our queries. that will be great blunder.