It has several names. The most common is probably hash. In North American English, it’s sometimes called the pound sign and used as a symbol for pounds weight: this can be confusing for British people for whom a pound sign is £. It’s also known as the number sign in North American English, in contexts such as go to question #2. In a musical context, the symbol is known as a sharp. The picturesque name octothorpe has also been introduced: it’s said to have been invented by an employee of Bell Laboratories in the 1960s, in honour of the American athlete Jim Thorpe (with the octo- part deriving from the symbol’s eight points). In the large form in which it appears on telephones it’s sometimes called a square.
Recently, the hash sign has acquired a new role. On social networking sites such as Twitter, it’s attached to keywords or phrases so as to identify messages on a particular topic (e.g. #volcano; #Iceland). These keywords or phrases are known as hashtags.