@ directs a post at someone, usually someone a comment is directed to or anybody you mention. @ precedes the handle.
# is called a hashtag. It is used to identify your topics, subjects and lists so other people can find your tweets, it works like a cross-reference or an index-tag. Note the spaces at the end of what you're @ or # and the lack of spaces in the middle of @ and #. Both symbols end at the next space.
@AmerigoVespucci Did you see @BarackObama 's interview with Robin Roberts where he supported #MarriageEquality ?
Some hashtags are used for lists. The only one I can recall right now is #tcot which is basically a teabagger extreme-conservative list. I generally tag all the the most offensive things I have to say with #tcot to piss off the loonies.
Chan790 in summary: Keynesian. Hawkish. Civil Libertarian. Liberal. Pro-Union. Supports Debt & Deficit Reduction. In favor of Scandinavian-style Social Democracy. Anti-Communist. Anti-Corporatist. Anti-Fascist. Anti-Free-Trade. Anti-Privatization.