Should "tags" be words or phrases?

[andrewmoquin]

A couple of questions about using tags ... should they be words or phrases? And how many should you use? I know it depends on the post and it's content ... but as a rule of thumb is it better to have more or less? Thanks!

[Zethix]

I think that tags should be concise phrases. Around 5 tags works best.

[melo030ui]

wow! so many posts about this subject. really give us useful information.