A desktop environment is a set of tools and other components bundled in an integrated manner that together provide a cohesive desktop experience using a common GUI. The live images available with Void Linux already come with some of these DEs built-in, but more are available in the form of regular packages that can be installed separately with XBPS. Just like the DEs in the live images, those available as separate downloads also vary in terms of features, requirements and aesthetics. Check out the related article in Wikipedia for more info about desktop environments as well as a gallery displaying many of the choices available.
As for Void Linux, here are a few DEs that can be individually installed with XBPS. Please note that this list is not necessarily exhaustive.