Don't know if it's been answered specifically, although the description is on the wiki page (see first post).
Anyway, Guardians are basically dwarven Golems. They can be gotten from the mountainhomes, either from the traders or as pets of your migrants (which is where yours came from). They're pretty good defenders, able to take on a single enemy pretty well. They get mulched pretty fast by entire groups, but can hold them off long enough for the dwarf to get away, usually.
Felsite guardians are the basic ones. Diorite guardians are tougher, and Obsidian guardians can dish out more damage.