They're both the exact same thing when it comes to exchanges. I was working for a betting exchange and a market maker for a while, taking 500 orders a second and arbitraging between different competitors, before moving back to finance. It's quite interesting actually.

A lot of the market is not tradable though. On one end of the market, gambling is often a state monopoly with high spread and high margins (if not prohibited entirely), while on the other end there are large trades going on in private or closed circles.