English nouns, verbs, adjectives and adverbs are organized into synonym sets, each representing one underlying lexical concept.
Different relations link the synonym sets.
I worked a lot with WordNet during my studies and used it as part of the research for my thesis. I use this gigantic database of English words and relationships everyday with as much pleasure as the first time. You can download it for free at the aforementioned site or look up words online. I coded a WAP interface that allows you to get the WordNet information you want while on the road. WapWord retrieves the possible word classes and selection choices.