Hello Shivani
I had to look this one up as black holes are not my specialty. The current theory is that there are 3 types of black holes; stellar, super massive and miniature. Stellar black holes form when a massive star collapses. Super massive black hole are likely at the center of most galaxies including our own. Miniature black holes have not yet been observed, but theory states they could have formed shortly after the Big Bang.
You can learn more about black holes at the website below
http://hubblesite.org/ref...php.id=62&cat=exotic
Dawn