What does the hash mean when written like '#this' ?

[STH]

Ive noticed recently that things are being written with a hash sign in front of them and not tv adverts have the slogan #with a hash before it.

What does it mean?

thank you

[Pete]

Twitter! When you hashtag something! Eg #xfactor or @xfactor
So it's when an event or product or trend or anything has a twitter page!

[Alex]

Its like a hashtag on twitter. If its on an advert then the company producing that advert probably checks that hashtag on twitter to see what peoples responces and thoughts about it are. Some people also use it in text as if they were on twitter, i.e. "i woke up at four am #tired"

[Alex]

It is a "hash tag" for Twitter.
Advertisers and promoters want you to notice their adverts and hope that the viewers will then go and tweet about it.
If it is an exceptional advert it may start trending.

[Whyburn]

So I am safe from those advertisers and promoters!
For I'll certainly ignore what is/are written after those hash signs, and I suppose it's just me, and my complaints would be ignored by them...