What does the @ (at) sign mean on Twitter ?

[insure_and_go]

i've watched twitter videos on youtube and done searches to get an explanation on google....but i still don't understand what the @ sign means on Twitter.

Can you explain it to me in straightforward language please?

Thanks !
...thanks Jeff P....but why do you have to @Twittername that person.......surely they will see your tweet anyway since they are already following you and you are following them
...sorry but i still dont get it

[Jeff P]

When you include @TwitterName, it lets that person know you have included them in a tweet. It's like sending a message to the user--but it's not private.

[BridgetC]

In some ways its more to let other people know you are talking or responding to one person in particular. Kinda self-filtering your feed.
Because if, for example, I write:

"yes! I totally agree!"
then all my other followers have no idea what I'm talking about.
If I write:
@sue yes! I totally agree!
then everyone knows I'm agreeing with sue- and if they aren't friended to sue they can either ignore my tweet or go to sue's page to see if whatever I agreed with was cool and they want to add her as a follower.

Also- the "yes! I totally agree!" doesn't let sue know if I'm agreeing with her or with someone else- because maybe a bunch of my followers just gave great advice- so I need to specify who I'm agreeing with.

It's like if you're in a big room of people and you say something without context to someone across the room, you probably say the person's name so everyone knows who you're addressing.

It also provides a link to their stream, so you can use it to reference someone, also.