Might have a serial number or something similar you need to enter, with a checksum to verify it's valid. Lots of shareware titles do this. The demo is actually the full game, with just a simple check to see whether or not you're allowed to access all content, or play for longer than 30 minutes at a time, or level up your character past level 10, etc. Not every form of copy-protection is DRM, but if you don't have a legitimate copy you'd still need to bypass it in order to play the full game.
Even bypassing something as simple as a dialog that prompts you to "Type the magic word" to continue, with "the magic word" being what you have to type would qualify as a "crack".