what does the # and @ mean on Twitter?

[name.]

= hash tag,
basically what it is, is if there is a current trending topic (trends listed on the right hand side of your homepage) it is just kind of to tag that that's what their tweet is referring to. or in some cases, i use the # to write stuff like "#iamcool" (example, i don't actually do that) so that when i click on it, i can see who else has wrote it.

@ = reply/mention,
this means you can put it infront of someones name "@username" and type them a tweet and it will automatically go to their mentions so you can have a conversation with them.