Are yellow bass good or bad for a pond (1/2acre). And what do they look like ?? Do they have a brown dot on their gill ??

Picture of yellow bass - Taken by Konrad Schmidt, this guy knows his fish.
http://www.gen.umn.edu/research/fish/fishes/yellow_bass.html

Fishbase doesn't have as much info for YBass as it does for other species, but its always a good place to start looking.
http://www.fishbase.org/Summary/SpeciesSummary.cfm?genusname=Morone&speciesname=mississippiensis

If that is yellow bass what are yellow perch?? BTW, I here that Large Mouth Bass are really part of the perch family and not the bass family. Is this true?

Rgreer -- yellow bass are a smaller cousin of the white bass. I'd guess they rarely get more than about 8 inches long, and that would probably be a big one. They will really eat your small forage fishes, but not contribute much to your sport fishery. Unless someone with more experience can give you a "positive" side to them, I think that I'd keep them out of a pond.

Big Pond -- Yellow bass, white bass, striped bass, and the white perch are all cousins (Percichthyidae, if you care about family names).

The largemouth bass is a member of the sunfish family, with smallmouth bass, spotted bass, the two crappies, bluegills, green sunfish, redear sunfish (shellcrackers), rock bass, etc. (Centrarchidae).

The yellow perch is a smaller cousin of the walleye and sauger (Percidae).

Common and local names can really get confusing. When I lived in Kansas, the green sunfish was locally called "perch," while they called the few yellow perch in the state "ring perch." I'm sure most everyone has more stories of confusing common names. That's why the American Fisheries Society keeps a list of the "official" common names for each fish species.

Hope that not too much information!! :-)

Dave