The Deweys are three neighborhood boys who live with Eva. Although they look very different from each other when they first arrive, everyone starts to treat them like a single entity, and soon no one can tell them apart. The Deweys are involved in the tunnel collapse at the end of the novel, and everyone assumes that they die, but no one ever finds their bodies.
Tar Baby is a white alcoholic who keeps to himself. He's quiet and introverted, but he doesn't bother anyone. He also dies in the tunnel accident, which probably suits him just fine, since all he wants from Eva's house is a place to die.
Why are these characters important? Well, these are the people who surround Sula as she's growing up, sharing her home and table. They aren't the most emotionally stable bunch, and this likely has an impact on Sula. They are perhaps one reason she loves Nel's house so much. There aren't any neighborhood boys or drunks there, and Sula can just enjoy the peace and quiet.