Jack Frost is The Graveyard Book’s arch-villain. He’s the guy who butchers Bod’s family with a sharp knife, and he wants to do the same to Bod. He also, it seems, lives with his grandma.
As we learn part way through the book, the Jack that killed Bod's family isn't the only evil Jack out there. The Jacks of All Trades are a top-secret group. They've been around for thousands of years. It's an organization called brotherhood, or fraternal order. In other words, no girls allowed.
We don’t know if they had other names before they became Jacks, or if they were born Jacks. They definitely don’t seem like the storybook Jacks some of them are named after. Jack Frost of storybooks is the guy who brings winter, not the guy who kills babies. Their nursery-rhyme-sounding names add to their spooky, mysterious quality.
Speaking of mystery, we aren’t quite sure what kind of creatures the Jacks are. They’re definitely people of some sort, but the novel identifies all the characters as people. They’re not dead people, for sure. They don’t seem to have any super-human physical abilities, other than maybe a sharp sense of smell. They use weapons to kill and have to climb over stuff just to get into the graveyard, just like the rest of us. We don’t know for sure if they can be killed or die in the same ways you and I can.
More importantly, it’s not at all clear who their other victims are, or what their crimes are (other than killing Bod’s family). In the Interlude, we see that the Jack do lots of good deeds, or at least pretend to, probably to cover up their evil activities. We also see that they have the appearance of being respectable gentlemen, meaning they wear expensive suits and have nice haircuts. They seem wealthy. But, mostly they remain mysterious. And we think that’s just fine – part of the fun of The Graveyard Book is that many things simply aren’t explained. We get to guess and try to dig around for clues.
Silas and the Honor Guard seem to have been fighting the Jacks for a while, but it's Bod who ends the group for good. In Chapter 7 we find out from Jack Dandy that thousands of years ago, someone predicted that Bod (or someone a lot like Bod) would destroy the Jacks:
“Long time ago, one of our people – that was back in Egypt, in pyramid days – he foresaw that one day, there would be a child born would walk the borderland of the living and the dead. That if this child grew to adulthood it would mean the end of our order and all that we stand for.” (7.605)
When Bod fights the Jacks in the graveyard at the end of the book, he finishes them off.