A revenant. Okay, you'll need a casket and some silver spikes. Oh, and don't get bit. No, it won't turn you, but it will hurt like hell. Okay, so, once you got all that, nail that sucker in, bury him, and throw away the key.
A revenant is a visible ghost or animated corpse that returns to terrorize the living, usually people it knew in life. Unlike the zombie, a revenant is not under another person's control, and has returned from the grave on its own for a specific and often personal purpose. Revenants react to silver and can only be stopped by nailing them into their coffins with silver spikes.
After Dean is resurrected, he makes his way to Bobby's. When Dean approaches him, Bobby lunges forward with a silver knife. Dean evades him and tries to convince him that he is the real Dean by telling him information only he would know. Bobby approaches Dean and tries to cut him again, with Dean telling him isn't a shapeshifter. Bobby goes on to accuse him of being a revenant, so Dean takes the silver knife and cuts his arm, proving to Bobby he is neither a shifter nor revenant.
When Sam and Dean learn that Bobby's wife has come back from the dead, they question him as to whether she is a zombie or revenant. Bobby, however, is just as confused, telling Sam and Dean that she has had no reactions to salt, silver, or holy water.
Ending up on the same case as Sam and Dean, Garth takes a call from a hunter named Earl asking how to deal with a revenant. Garth tells him to get a casket and some silver spikes, then nail it in and bury it. He also warns that getting bit will hurt like hell, but will not turn you into one.
Sam and Dean take on a case where the deceased wife of the victim, Wes Bailey, appeared to have stabbed her husband and walked out the front door. Confused as to why a ghost would leave that way, and the nothing registering on the EMF, a revenant is suggested. When Jack questions what a revenant is, Sam tells him it is like a zombie.
When their EMF meter shows no signs of ghost activity at the second crime scenes, Sam and Dean become convinced they are dealing with revenants, and head to the cemetery to dig up the grave of Erica Bailey. Once dug up, they find the body of Erica still in the coffin, taking their theory back to a ghost, prompting Dean to salt and burn Erica's body.