One of the best places that I've found for not only getting definitions but additional related information is Webopedia.
If you have specific question the members here will be more than willing to share their knowledge.
As for what makes a good game machine, I'm not a gamer, but I think most would agree that there a few basics that are a must. A good PCI-E graphics card with 512MB cache, at least one gig of RAM (two x 512MB in dual channel), and a PSU large enough to fulfill the requirements of the graphics card...they can be power hungry. So far dual core processors aren't playing that big of a roll in current games, but that looks like it will change in the future. I'm sure that others here will add more to this short list.