Dual core vs single core...?

What's the deal with dual core? I mean, what are it's advantages verses single core? A guy from work tried telling me it makes the processor twice as fast as it's stated GHz (2.2 = 4.4) but I don't think thats right... I have a 2.16 GHz core 2 duo in my MacBook and it's just slightly faster encoding a movie file than my father-in-laws 2.6 GHz single core Dell desktop so it can't be "twice" as fast. And if you have time, what's the deal with quad core? And if you have more time (sorry for so many questions), what about dual processors (dual & quad)-what exactly does that do? I've been trying to talk my father-in-law into replacing his 3 desktops (2 Windows XP's and 1 Windows 98) with 1 Mac Pro (dual quad core)... I'm no computer genius but I'm thinking it's very fast. He does alot of video editing. Anyhoo, if you know alot about this sort of thing, please explain it to me. Thanx in advance.