Web development broadly refers to the tasks associated with developing websites for hosting via intranet or internet. The web development process includes web design, web content development, client-side/server-side scripting and network security configuration, among other tasks.

Front-End: This is the User Interface, the part you actually see and interact with. HTML, CSS, and Javascript are the core technologies used to develop front-end. As you must have noticed, HTML/CSS/Javascript files are served by a web server and are rendered by browser(client) to display the UI. These files can be served statically or it can be generated dynamically using BackEnd technologies.

FRONT END

The fundamentals of HTML,CSS, Javascript, which is all you basically need to develop front-end, you may want to learn some other related technologies. You can do without these technologies but these will make your web development task a lot more easy. They are :

a) javascript libraries : jquery/ExtJS/YUI/etc. These libraries make working with javascript and ajax a lot more easy.
b) Javascript frameworks : angular.js/backbone.js/sproutcore/etc. Advanced MVC frameworks for projects that use a lot of javascript.
c) frontend frameworks : twitter bootstrap/foundation. Learn any one.
d) CSS preprocessors : LESS/SASS. Adds new capabilities to css like variables, mixins, etc.
e) Coffeescript : A language that compiles to javascript.

BACK-END

This is where the core business logic resides. Typically a back-end application consists of a programming language, a framework and a database. Its role is to perform the business logic on server, save the information in database and send the changes in UI to client as HTML. If the backend is a REST API it can serve JSON/XML instead of HTML and Javascript can update HTML after parsing JSON/XML. You have many options when it comes to language / frameworks for backend and you can choose any one according to your linking.

PHP + CodeIgniter/Yii/Symfony/etc.
Python + Django/Flask/etc.
Ruby + Rails/Sinatra/etc. (but you can also learn this)
C# + ASP .net MVC
Javascript(node.js) + Express framework
Databases used are either relational (MySQL, PostgreSQL, SQL Server, etc