A gas ram is basically a pneumatic ram that fits in place of a spring. To all intents and purposes, it works just like a spring - you don't need a gas cylinder - you simply cock and reload. The main difference from a user's perspective is the firing cycle. In many ways, it's similar to a rifle - a short, sharp, shock. Many hate this feeling. I love my gas ram guns though (but then I also love my PCPs and my springers!). The only way to be sure is to try one and see if you like it.

Nealsey - you beat me to it!