That's a real big question, Grimm, since the Daleks have been going since 1963 and built up a huge history over the years (which often contradicts itelf). To Brits of a certain age, Doctor Who and the Daleks are an ingrained part of our culture. I grew up with them, they were a cherished part of my childhood, and I still think they are really cool. At its height in the 1960s the "DALEKMANIA" craze was almost as big as "BEATLEMANIA."
Rather than try and tell you all about them myself, the lazy thing to do would be for me to just link to the Wikipedia entry on them, so that's what I'll do!
http://en.wikipedia.org/wiki/DalekHere's quite a cool clip from a recent series that shows one in action (they weren't always this cool):
And just for Grimm, a German Dalek:
I wonder why they gave the German Dalek such a silly baby voice? Doesn't really inspire much terror, does it?
Finally, a musical montage of the Daleks in action over the years: