Creatine is not a steroid. It can enhance physical performance in certain contexts, but it is not a steroid chemically, nor does it do anything to the human body that is even reminiscent of steroids. It can enhance the function of the muscle you already have and even theoretically make it a little bit easier to make new muscle... but the same could be said for eating lots of steak and eggs. In fact, steak has a lot of creatine in it... creatine supplements just make it much easier to consume a bunch of creatine at a time. Steroids are different--much more like taking a "drug" in a lot of ways. Plus they are much more harmful... and the better they work, the more harmful they are, generally speaking.
You're right that it can be tricky sometimes to tell the difference between a "drug" and a "supplement", but creatine and steroids are pretty clear-cut, in my opinion.
Almost all prohormones are now illegal too--prohormones are, actually, technically steroids (which is a chemical term), but most of them aren't considered anabolic steroids (which is the class of steroids that we are talking about), and none of the legal ones I know of are significantly anabolic. The only prohormones commonly available don't do very much if anything.
