My Dog and Cat Hate each other?

My recently adopted terrier mix, Bentley, gets along just fine with our female cat, Cricket. But he and my male cat Sammy, get into fights...sorta, they really only just make noise at each other, but sometimes they smack each other. is this just an alpha thing? a male to male thing? cat to dog thing? what? and how can i get them to stop and start getting along? please help...