It's essentially a PC programming language that features many of the structures and commands found in the traditional BASIC language. You may know of other BASICs like QBASIC or Visual BASIC, well DarkBASIC is different and unique because it offers powerful game specific commands that can control 3D objects, vast landscape terrains, sprites, sound, music, multiplayer and much more.
DarkBASIC has been designed for both beginners and professionals. Its aim is to provide an easy way to create PC computer games and at the same time offer powerful commands that are versatile and varied.
The question really should be "What can't I make with DarkBASIC?!"
Before DarkBASIC was available there was only really one route to making games on the PC. That route entailed learning C++ and delving into the software development kit for Microsofts Direct X engine. In simple terms, Direct X is an interface which communictes directly to your video and sound hardware. It saves developers the task of having to code for every type of video and sound card out there, instead all they have to do is learn how to command Direct X - but even this is tricky for most users. DarkBASIC does this by giving you really easy to understand commands. It takes these commands and then communicates your intentions to Direct X.
DarkBASIC is a programming language, this means you are not restricted in what you can create. You can choose to code any game type you like, examples done in the past include: multiplayer, FPS, card games, driving games, puzzle games and many more besides.