line 6 spider II. good amp, really cheap. you can get a 15W version (plenty for practicing) for like $160. it's a great amp, good cleans, and the "insane" channel is great for playing metal.

head and cabinet:
head: the part that actually takes the signal from the guitar, and shapes the sound and amplifies it

cabinet: the speakers that the head puts that sound that it has amplified through, allowing you to audibly hear the sound coming from the head

combo amp: both the amplifier and speaker(s) in one box, this is undoubtedly what you are using right now, and is great for a practice amp, and certain ones (depending on the wattage and speaker size) are more then loud enough for gigs. a half stack (another name for having both a head and cabinet in seperate pieces, a full stack is a head and two cabinets) is a bit of overkill unless you are playing really loud music for (at least) a few hundred people