Although slang tends to vary from state to state and country to country, some words are fairly universal in describing the quality of weed.
Schwag: This is the worst grade of weed. Brown, seedy, and usually dry. It is very cheap, but you get what you pay for in this case.
Mids: This is the middle of the road weed. You'll start seeing some nug forms and possibly some red hairs and crystals. This weed will be of a lighter color and much prettier than schwag. Few seeds usually in mids.
Dank: This is sizeable nugs of weed usually. Plenty of red hairs, a nice green color and no seeds are attributes of dank. Most "dank" that I've encountered seems to be beasters. Beaster is the low end of the high-class dank weed. It is commercially grown, seems mostly indica. Definitely not a designer specific strain.
Headies: What is called "heads" or "headies" is usually a very high quality weed - usually one of the more exotic strains. It is also the most expensive.