Why are those little hashtags in sheet music?

[☢ ASOT600 ☢]

It means a note is 'sharp'. Let's take the note 'C'. 'C#' means (apart from being a programming language) that you play the BLACK note, ONE TO THE RIGHT of the standard 'C' white note. Here's a nice diagram:

http://www.piano-keyboard-guide.com/imag...gram_2.JPG

Notice that sharp notes (#) are always one to the RIGHT, whereas flat notes (b) are one to the LEFT. Obviously a C# is the same as a D flat. If you look at C flat though (ie the black note one to the left of white C), there isn't one. There's a gap.