Hi Wanda,
Common goldfish have shorter fins and comets have long fins.  that's pretty much the difference between the two, as both are slim bodied shape and fast swimmers.

Goldfish will eat just about everything.   They would enjoy the bloodworms and if you want to try the recipe on this site go ahead.  I think most of us feed frozen peas popped out of their skin and bits of orange as a treat. 

Goldfish treat most live aquatic plants like we treat potato chips...
The love to munch on them!  Make sure you place only aquatic plants that your pet shop approves as safe for aquariums since some common regular house plants may be poisoness.
Good luck with your new little friend!

Our fish also have a clip on a suction cup designed for aquariums.  You can put lettuce, algae wafers, and other munchies in that, and it's easy to take out the leftovers.  I like goldfish granules myself because they sink and the fishies don't gulp as much air.  Also, don't freak out if they go after a piece of their own poop!  They will spit it out 99.9% of the time; fish just go after floating things or things on top of the water in general, such as air bubbles, pieces of plant, and, once, my little finger.

I can't add whats already been mentioned for different fish foods, all I can say is their stomachs are very very hardy.. like, have you ever seen a fish.. you know.. throw up? I dont think they have a gag reflex.. lol