Good beer is determined by an individuals tastes. It has been suggested that trying a wide variety of beers will usually help a person figure out what beer tastes good. Bad beer is beer that tastes bad of is spoiled. Beer can and will spoil under certain conditions. Mishandling and old age are the two biggest causes of spoiled beer.
Skunked beer refers to beer that has been lightstruck, causing the hops to take on a skunky odor.
This is often happens with clear or green bottles, and tends to be prevalent in certain imported beers. Good books on beer Michael Jackson's Beer Companion does not give ratings. It discusses various styles and profiles good examples of the the styles. This is a very enjoyable book that every beer lover should have. About 1300 beers from around the world are reviewed and rated.