why do people put a # before a word?

[Spudoodle]

for example #cool
idk if i did it right, but hope u know what i mean
hash?

[chels]

it's a hash tag

[Everyday Cynic]

Some dumb thing from twitter that people are bringing to all the other websites.

[Mr Chill]

Which age group are you referring to, I never have done so.

[Tina]

i haven't ever seen anyone put a number sign in front of a word, i guess my friends are too old for that

[Cheesy Niblets]

it's a hash tag, or just a "tag." Basically you put it in front of a word and on social networking places like twitter it will show it in the relevant area, for example if I was talking about...horses, and at the end of my post I put #horses, it means if you searched on a social networking site, it would come up with other, relevant posts. It's just a way of connecting with people who have the same interests as you and broadcasting your posts/tweets and stuff, to a relevant audience.
Hope I helped!