Hey I stumbled across this today....http://en.wikipedia.org/wiki/Minor_Dark_wizards_in_Harry_Potter
It is really informative and there are links to a J.K Rowling interview and other links to characters from all the books. There may be some spoilers in there so be careful. ;)
Wow that’s really interesting! I’m still going through it all. Thanks for the link!
Grindelwald was a Dark wizard before Lord Voldemort and his only mention is in Harry Potter and the Philosopher's Stone on Albus Dumbledore's Chocolate Frog card which notes that Dumbledore defeated Grindelwald in 1945.
Rowling apparently once claimed that there was a connection between Grindelwald and Adolf Hitler (hence the 1945 date of defeat); she has stated that she likes to imagine a connection between the wizarding world and the real one.[2]
Grindelwald has something to do with the remaining plot. [3].
Grindelwald is a Germanic name: Wald means "Forest", and Grindelwald is a ski resort in Switzerland. The name Grindelwald also bears a resemblance to the mythic Grendel who was defeated by Beowulf. It is worth noting that one of Dumbledore's middle names is Wulfric - close to Beowulf. Given Rowling's penchant for clever naming, it is likely that this was intended.