Does anyone know what this symbol means?

On my keyboard it is the shifted character on the leftmost key of the top (numerals) row. Until you asked, I never knew what it was for but, apparently, it is a logical "not" sign and its official name is "not". In Boolean logic, something may be either TRUE or FALSE (this is actually one meaning of the "0"s and "1"s which computers use all the time). Something which is TRUE is not FALSE and something which is FALSE is not TRUE ... which may seem too obvious to be worth stating, of course, but there are various things you can do with logic which I won't go into here. Anyway, it is useful for mathematicians and programmers to have a sign which tells you that something is NOT something else, just as it's useful to have a sign ("+") to tell you to add two numbers together. I hope that has explained somewhat what "¬" is for. In other words, basically useless for 99.9% of people or "¬ for me!"

It doesn't seem familiar to me, but could you describe where you found it, was it in a text? I might have seen it somewhere.. I am curious as well :)