Before I dive into what LibGDX is, here is how LibGDX describes itself. From the LibGDX wiki—https://github.com/libgdx/libgdx/wiki/Introduction:
LibGDX is a cross-platform game and visualization development framework.
So what does that actually mean? What can LibGDX do for us game-makers that allows us to focus purely on the gameplay?
To begin with, LibGDX is Java-based. This means you can reuse a lot, and I mean a lot, of tools that already exist in the Java world.
I can imagine a few of you right now must be thinking, "But Java? For a game? I thought Java is supposed to be slow". To a certain extent, this can be true; after all, Java is still an interpreted language that runs in a virtual machine. However, to combat the need for the best possible performance, LibGDX takes advantage of the Java Native Interface (JNI) to implement native platform code and negate the performance disadvantage. One of the beauties of LibGDX is that it allows you to go as low-level as you would like. Direct access to filesystems, input devices, audio devices, and OpenGL (via OpenGL ES 2.0/3.0) is provided. However, the added edge LibGDX gives is that with the APIs that are built on top of these low-level facilities, displaying an image on the screen takes now a days only a few lines of code.
Note
A full list of the available features for LibGDX can be found here: http://libgdx.badlogicgames.com/features.html
I am happy to wait here while you go and check it out.
Impressive list of features, no?
So, how cross-platform is this gaming platform? This is probably what you are thinking now. Well, as mentioned before, games are being delivered on many different platforms, be it consoles, PCs, or mobiles.
LibGDX currently supports the following platforms:
Windows
Linux
Mac OS X
Android
BlackBerry
iOS
HTML/WebGL
That is a pretty comprehensive list. Being able to write your game once and have it delivered to all the preceding platforms is pretty powerful.
At this point, I would like to mention that LibGDX is completely free and open source. You can go to https://github.com/libGDX/libGDX and check out all the code in all its glory. If the code does something and you would like to understand how, it is all possible; or, if you find a bug, you can make a fix and offer it back to the community.
Along with the source code, there are plenty of tests and demos showcasing what LibGDX can do, and more importantly, how to do it. Check out the wiki for more information:
"Who else uses LibGDX?" is quite a common query that comes up during a LibGDX discussion. Well it turns out just about everyone has used it. Google released a game called "Ingress" (https://play.google.com/store/apps/details?id=com.nianticproject.ingress&hl=en) on the play store in 2013, which uses LibGDX. Even Intel (https://software.intel.com/en-us/articles/getting-started-with-libgdx-a-cross-platform-game-development-framework) has shown an interest in LibGDX. Finally, I would like to end this section with another quote from the LibGDX website:
LibGDX aims to be a framework rather than an engine, acknowledging that there is no one-size-fits-all solution. Instead we give you powerful abstractions that let you chose how you want to write your game or application.
LibGDX wiki—https://github.com/libgdx/libgdx/wiki/Introduction
This means that you can use the available tools if you want to; if not, you can dive deeper into the framework and create your own!