But more than that, they are different styles of living. Campo would generally mean you live in a cortijo or finca (traditional old Spanish rural property), perhaps quite isolated, probably up a 2 or 3 kilometre track from the nearest surfaced road, or that you live in a newish villa out in the country, probably with amazing views and a pool, but probably with high gates for safety. Very quiet.
Pueblo, as Gus says, is of variable size - somewhere between a village and a town! My pueblo has 6 bakeries, 18 bars, a good DIY shop, 2 banks, a post office, and a reasonably efficient town hall, all in friendly walking distance. Estate agents will often describe properties as "village houses", and they will be mostly traditional Spanish houses in a terraced street, and you are likely to be surrounded by Spanish neighbours, kids, late-night chatter in the summer, and some dog & motorbike noise.
Urbanisation - a whole way of life! I'm no expert on this, but you might want to look in the "Communities" section of Eye on Spain to read about the way different communities / urbanisations organise themselves. Different to anything in the UK, they have a President and levy quite high annual charges (obviously you get services for this, such as shared pool, gardening, etc). There seem to me to be lots of threads on the forum about problems with Presidents and community fees, and there is a whole chunk of Spanish law relating to this. Your neighbours are more likely to be other expats.
So what you choose depends on many things - how much Spanish you speak, whether you are bringing kids, whether you want to hang out with other Brits, whether you want the convenience of a shared pool etc, whether you want the quiet of the countryside, the bustle of a village, whatever.
Apart from my blog, which is about my recent purchase in a village and how that's going, I would also suggest you look at Eggcup's blog as a warning about some of the pitfalls of making a wrong choice, and at Gerald's blog as he is still in the house-hunting stage. Between the three of us we pretty much cover lots of information!
Best of luck - I do think you have hit the nail on the head with your first question. I think TYPE of living is the first choice, area can follow afterwards.