On a phone keypad it is known as the pound key (US) or hash key in most English-speaking countries.
When # precedes a number it is number as in #1 (number 1). When it follows a number it is pound (unit of weight) as in 3# (three pounds) (primarily US).
Other names for the # include hex and octothorp. # may be used in computing programming, math, Web pages (such as shorthand for the permalink of a blog or to denote a special tag such as the hashtag on Twitter), chess, and copywriting. Three pound signs (###) often denotes "the end" in press releases or typed manuscripts.
On standard US keyboards the # key is Shift+3. It may be in other locations in other countries. Mac: Option+3. Windows: ALT+35
Although the musical notation for sharp (♯) looks similar, it is not the same as the number sign. The number sign generally consists of 2 (usually) horizontal lines and 2 forward slashes. Whereas, the sharp is 2 vertical lines and 2 tilted lines so that it appears to be leaning back to the left while the number sign is more upright or leaning to the right.