What divides London into East, West, North and South?

The other day whilst I got my make-up (I'd rather not spend another thought on this) I had a quite interesting discussion with the stylist about the best places to live in London. East, West? North or south the river? Spontaneously I'd say: SW - South West.

And that's not surprising when keeping in mind that my first place in London was in Fulham. And now one year later I'm still living there (one house further down the road though). But when asking others they will probably say: the East End, North London, south of the river. So I started thinking about what the actual differences are? It's all one city, a quite big city spreading over an awful big area though. Here's my conclusion: the people. And where they currently live. Lots of people in the West keep believing that they're living in the best area of London whereas the East and North are poor and quite rough, wicked and trendy though. Loads of people living in the East and North wouldn't want to move away from there but do still believe that the West is kind of a true coming dream although it's too posh, basically. And the South? Nobody north of the river would seriously consider moving south of the river and just a few people living in the South would want to relocate to the North. I personally believe that some boroughs south of the river are quite great places for living. Anyway. 

So this guy (he relocated to London from Cologne a couple of years ago) asked me where about in London I'm living (I like that he wouldn't consider me to live somewhere outside of London :). I replied in Fulham whereupon he just dropped the hint: Oh, in the West. And I said, No, in South-West London! And I bloody like it there! And then I asked him the same question. He just said, in the East, in Hackney and I wouldn't consider moving away from there. Because I like it there! And I just replied: Nice! Not that I would have been in Hackney before. I don't know anything about this part of London but I keep believing that it's rougher over there, but cooler and trendier than round here in the West.