Html is the language that is used to put content on the Web pages. CSS is what makes it look nice. CSS can also be used to ensure that all pages on a site (say a corporate site) all look the same by being able to use it's information on all html pages. More on reddit.com

HTML is a language designed to describe what a web page looks like. It looks as follows: this text becomes bold this text links to google If you use chrome, you can see a page's HTML code by right-clicking and 'inspect element' More on reddit.com

Since this is Explain like I'm five: What javascript is As you said, HTML is for content and CSS is for styling. Javascript, however, is a programming language. When used in the browser, it's mostly used to make things fancy and add functionality that would otherwise require a full reload from the page. (Javascript is not used exclusively on the browser, it's a programming language that can do everything any other language like python or C++ can, and in fact, although it has many flaws, it's a pretty cool language for web programming). How javascript is used So where's javascript used on reddit? If reddit existed in 1995, before javascript came along, clicking the save button to post this comment would send a POST HTTP request to reddit and the page would reload. Even if I could send the POST request without reloading somehow, my version of the webpage would not change (the comment would not appear), since there would be no way to update the HTML of the page dynamically while the user has the page open. There would be no other way to get content from the server. However, now that javascript exists, clicking save to post will not submit a request directly. Instead, it will trigger some javascript code that is executed within the browser. This piece of javascript can do pretty much everything with the data. In this case, it will check that the comment textbox is not empty, if it's not it will take it's content and send it to the server without reloading the page. The server (and I will explain why/how later) will receive the request, process it in some way, save it in some kind of database and return a reply that will hopefully be along the lines of "Ok, I saved the comment, kkthxbb". The javascript client will receive this reply and update your version of the webpage by closing the textbox and making the comment visible. What else is needed Of course HTML/CSS/JS is not all that's required in making a website. What's this server thing? Well, if you're going to have persistent content and many users in your application, you need to somehow save data somewhere. In reddit's case, links, posts, comments and up/down votes. That happens in a database, a program responsible for keeping data and performing operations on the data (updates, lookups, deletions) fast, really fast. The rest of the application interacts with this database, performing operations like "Add an new comment to the comments table and keep a reference to the containing post". Clearly, the users should be able to somehow access this database. I need some kind of access to reddit's database to post this comment. Why javascript isn't enough So why can't client-side javascript handle that? It can! Theoretically you could have a dynamic app like reddit that runs entirely on the browser. Database operations and data validation (ensuring my comment is not a a quazillion bytes long and doesn't contain any harmful code) would be done entirely on the user's browser. But that wouldn't be a very clever thing to do, since javascript code is visible to the user (there's no way to prevent this effectively since it's executed on the user's computer). On chrome, pressing ctrl+alt+I (if I remember correctly) let's you see all of reddit's javascript, and run your own javascript commands. That means it would be redicioulously easy to bypass any kind of validation and post whatever I want on the comments, even harmful code. Even worse, since the client would have read/write access to the database anybody would be able to delete and update anything. Clearly we don't want that. Whatever kind of app you're building, you'll almost always want to limit database access and validate user data in a safe way, away from the client. That's why there's a server application running on reddit's servers. That application has complete read/write access to the database and performs data validation, however users can't interact with it directly. You can only send requests like "POST a new comment" with arguments (the comment itself in this case) to the server. The server code (for reddit it's written in python) will run in an entirely different computer, process and validate your request and send you an answer. You could say that the server-side code is a middleman between you and the database. It's important to realize that the choice of language has nothing to do with safety. Performing data validation with javascript is safe, as long as you're not doing it on the client side. Javascript also works on the server-side. It depends on where the code is run, not in what language it's written. Conclusion Additionally, a website may have all kinds of code running on the backend. Reddit has an algorithm that chooses what posts will go to the front page and what posts will not. Usually when writing an web app, you use an existing database program like MySQL, CouchDB, Redis or MongoDB to store your data and perform operations on it. Then you have to write the backend code that respons to user requests, processes data and interacts with both the database and the clients, and the HTML and CSS to render data on the client and optionally some client-side javascript to add some logic on the client-side. TL;DR: You need back-end code for security reasons. More on reddit.com

http://en.wikipedia.org/wiki/Cascading_Style_Sheets CSS is what gives the page color and design. The fonts, colors, buttons, where things are located...all in the CSS. You can view a subreddits stylesheet by appending "/stylesheet' to the end of a URL. For example, if you wanted to see the stylesheet changes the mods have made for r/pics , you would type in " http://reddit.com/r/pics/stylesheet " You can view certain elements pretty easily in Firefox with the Firebug add on, which allows you to hover over certain things and see what the code for it is, as well as changing it, or in Chrome by right clicking and choosing "Inspect Element". This helps you to learn different elements, changing them, playing with them, etc. Cheers. More on reddit.com