Besides other things, AngularJS is primarily used to build Single Page Apps (SPAs), so let us first understand its characteristics.

Single Page Apps are apps or websites wherein the entire site or app content loads within a single page. This essentially means that once the app or website is loaded in the browser, clicking on any link would not reload the entire page but would simply update certain sections within the main page itself. This gives users a very desktop-like feel while using an SPA.

Although SPAs have become very popular nowadays, the concept has been discussed as early as 2003, and the term Single Page App was coined in 2005.

Some of the technologies that play a predominant role in building SPAs are HTML, CSS, JavaScript, AJAX, and web services usually RESTful. Of these, JavaScript plays the most crucial role in building an SPA, so if you have been procrastinating on sharpening your JavaScript skills this would be the best time to get up and get started.

One of the fundamental differences in the way SPAs work against regular websites is the way the pages are built, which the user sees. Refer to the following diagram:

In traditional web applications that are built on the server-side technologies such as Java, PHP, and .NET, whenever a page is requested, the web server would make a request to the database, fetch the result of the query, then load the template, and dynamically generate the final page, which is sent down to the browser. As you can see here, the web server is doing all the heavy lifting, and as the traffic to the server increases, the web server becomes a bottleneck. This is why popular high-traffic sites need a lot of servers.

Single Page Apps, especially those built on JavaScript frameworks such as AngularJS work in a slightly different fashion. Refer to the following diagram:

In an SPA architecture, the entire template along with the HTML, JavaScript, and CSS is downloaded to the user's browser, so when a request is made, content is sent from the web server and the page is built on the client side on the user's browser. Here, the browser is doing the heavy lifting. In such an architecture, the web server is merely passing raw data and is not involved in building the pages. The pages are built on each user's browser and hence even if the traffic to the site increases, the server doesn't get overloaded, as it would have in a regular web app architecture.

Another thing that makes SPAs wonderful is that the presentation layer can be completely decoupled from the backend layer.