what does the "#" key mean?

[seth j]

Hello,

Over the past couple of years, I've seen alot of people (more do than dont) put in hastags "#" at the start of a sentence, what does it actually mean?

I've seen programmes on TV with it on, like, #TheXFactor, etc.

I dont do twitter or facebook or anything like that.
I already knew "#" meant number, as in number 3 - #3.

All this #TheXFactor etc, just doesn't mean anything to me.
#onedirection your the best. means this sentence is for them

Why not just go, @onedirection?

[Jenesis Farmerz]

means for those who do twitter. it's pathetic.

[kat]

That key means number. like #3. It is also called the pound or hash. #TheXFactor is an example of a hashtag which is popular on Twitter and Facebook.

[Filebuster]

Hash. Hash tag. Used to add context to the comment / sentence so that all comments with same hashtag can be aggregated and found together. #explanation #twitter

[tsukimi]

it is basically saying that this info is for someone or something or label attached to someone or something for identification or other information.
#onedirection your the best. means this sentence is for them

@ means place. like iam having so much fun with my friends @NY