You shouldn't really think of it as 'the stock exchange' - even though one particular company presently has the lion's share of trade.
Since ghostcheck mentioned "companies" it is probably safe to assume the GLBSE market is what is being referred to.
Hopefully, ghostcheck, that you take the time to read up to understand exactly what those "companies" are. None (or not most) are "companies" in any legal sense (i.e., not registered as a corporation or LLC). That doesn't mean you can't make money through "investing" in some of those projects, but just be sure to understand that this "stock exchange" is not the same as ones you might already be familiar with.