[MrPbody]

It all depends on what engine family you're talking about. The two most commonly refered to as "open" and "closed" are the big block Chevy and the Pontiac.
Early BBCs ('65-'67) are "closed" chamber. That is, the chamber is oval-shaped and the spark plug is exposed to the flame front on one side, only. The same is true of the Pontiac. In '68, they "openned" the chamber to allow for better flame propegation across the top of the piston. The main engineering rationale was to reduce emmisions, making the engine more fuel efficient at lower RPM. It worked. Pontiacs needed an AIR (Air Injection Reactor) pump to pass '67 federal standards. In '68, with the new chamber shape, the pump was no longer needed. It didn't do as much for cleaning up the Chevy, but it did significantly improve performance.
I've heard people refer to small block heads as open or closed, but that's not really accurate. Small blocks have "large" and "small" chambers. The 327 and early 350s had a small (64 CC or less) chamber in the performance engines, for more compression with a flat top piston. The lower level engines had "large" chambers (70 CC or more), to keep the compression down. There are a couple of variations with the early stuff. There are almost as many chamber designs as there are people making heads with the modern stuff.
Piston design for BBC varies between chamber types. Most others use flat tops, therefore, no difference until you get into racing heads and pistons. While you can use "closed chamber" pistons in an "open chamber" build with BBC, it isn't recommended as flame travel is degraded. You cannot use "open chamber" pistons with the "closed chamber" heads, as the bump on the piston will hit the head.
A picture is worth a thousand words, but I don't have any handy. I hope this helps some.