What do the # and the @ symbols mean in plain English on Twitter?

[Kelli]

I cannot figure this out. Help anyone?

[Tonii]

= Number
@ = At

I Think....

[Sami Apon]

means number
and
@ means at
in plain english to be specific

[Melanie]

You put @ before a person's username to reply to something that they said, or if you mention them in your twitter. On the right hand side of your home page you can click and see when people have used @yourusername and see if they've mentioned you or replied to your posts.

The # is for searches, so that when you search for the phrase in the search box (which is also on the righthand side) all the posts that include #thatphrase show up.