libwacom has been around since 2011 now but I'm still getting the odd question or surprise at what libwacom does, is, or should be. So here's a short summary:
libwacom only provides descriptions
libwacom is a library that provides tablet descriptions but no actual tablet event handling functionality. Simply said, it's a library that provides axes to a bunch of text files. Graphics tablets are complex and to integrate them well we usually need to know more about them than the information the kernel reports. If you need to know whether the tablet is a standalone one (Wacom Intuos series) or a built-in one (Wacom Cintiq series), libwacom will tell you that. You need to know how many LEDs and mode groups a tablet has? libwacom will tell you that. You need an SVG to draw a representation of the tablet's button layout? libwacom will give you that. You need to know which stylus is compatible with your tablet? libwacom knows about that too.
But that's all it does. You cannot feed events to libwacom, and it will not initialise the device for you. It just provides static device descriptions.
libwacom does not make your tablet work
If your tablet isn't working or the buttons aren't handled correctly, or the stylus is moving the wrong way, libwacom won't be able to help with that. As said above, it merely provides extra information about the device but is otherwise completely ignorant of the actual tablet.
libwacom handles any tablet
Sure, it's named after Wacom tablets because that's where the majority of effort goes (not least because Wacom employs Linux developers!). But the description format is independent of the brand so you can add non-Wacom tablets to it too.
Caveat: many of the cheap non-Wacom tablets re-use USB ids so two completely different devices would have the same USB ID, making a static device description useless.
Who uses libwacom?
Right now, the two most prevalent users of libwacom are GNOME and libinput. GNOME's control center and mutter use libwacom for tablet-to-screen mappings as well as to show the various stylus capabilities. And it uses the SVG to draw an overlay for pad buttons. libinput uses it to associate the LEDs on the pad with the right buttons and to initialise the stylus tools axes correctly. The kernel always exposes all possible axes on the event node but not all styli have all axes. With libwacom, we can initialise the stylus tool based on the correct information.
So now I expect you to say something like "Oh wow, I'm like totally excited about libwacom now and I want to know more and get involved!". Well, fear not, there is more information and links to the repos in the wiki.