Number sign is a name for the symbol #, which is used for a variety of purposes, including the designation of a number (for example, "#1" stands for "number one"). The symbol is defined in Unicode as U+0023 # number sign (HTML: # as in ASCII).
In Commonwealth English, the symbol is usually called the hash and the corresponding telephone key is called the hash key. In American and Canadian English, the symbol is usually called the pound sign (outside the US, this term often describes instead the British currency symbol "£") and the telephone key is called the pound key.
When used in technology, it is always referred to as hash, and when preceding a word or phrase on social networking sites is referred to as a hash tag.