OK Everyone, When Did These Symbols Change? And WHY?

Remember when # meant “Number?” As in #10? Or if you placed it elsewhere, it meant Pound (or Pounds), as in 10# of potatoes. Now, its suddenly something called a “Hashtag” for all the twits in the world who Tweeter or whatever the hell it’s called. So now you get #10@10# perhaps? What is a hashtag? I know what a number is…..know what a pound is, but where can I go to buy a hashtag of something or the other? This is vitally important, my life won’t be complete until I have a hashtag standing in the corner, or sitting there, collecting dust.