Troll - This seems to be one of the most misunderstood...I see people using troll whenever someone has anything flamish, or if they don't agree with what someone writes. Just because someone says Windows works better then Linux, this is not a Troll. That would be Flamebait, which will be covered later. Troll is someone like the GNAA folks, or any other comments that are specifically to hurt slashdot, say blatently fake facts, or article text with subversive content.
Flamebait - This is usually used correctly, use this if someone says something that is obviously meant to get people's dander up. Say being the first post in an evolution debate and saying that "only idiots believe in Creation and/or Evolution".
Redundant - If someone is the 100th person to say something...but really you should only use this if they have been unfairly modded up...no sense in wasting mod points in modding something from 0 to -1 just cuz you like being mean. Most people are too stupid to surf at anything besides 4 and 5 comments, so your better bringing a good comment up then using this. Also be reasonable. If someone posted something in the first 7 or so original level posts, they werent TRYING to be redundant.
Overrated - Someone made a comment like "YEah I agree, Google rocks!" and they are modded up to +5? Good time to use this puppy. Also if you don't want to be meta moderated I guess this is a good one, since these don't seem to get meta moderated.
Informative - If someone conveys new or useful information, say a link to a scientists theory on whatever, or a fact that he/she knows as an expert in the field. This is not for people's opinion. If someone says "Microsoft is evil" this isnt really informative, if you agree I guess that's more Insightful, but really comments like that don't deserve to be modded up
Insightful - This is for good OPINIONS, a link to the coral cache or whatever is not insightful, it's informative. Remember, Insightful is for opinions, Informative is for factual type stuff
Funny - Of course this is a grey area...but I see so many things that are not even intended to be Funny modded as funny. Like sometimes I get the modding "Exchange is the best mail server" comments funny, as sarcastic or something, but it's too deep to appreciate your humor at this level. If something is funny haha, not funny uh oh, use this.