To those familiar with K&K, I find the best way to describe Extract brewing is to say:
With K&K you pour in 2L of boiling water to dissolve the can goo and the sugaz. With extract brewing that boiling water has your hop bittering and flavour in it - and probably some colour and grain flavour and body. Everything else remains the same.
It's exactly the same as K&K except a bit of effort goes into that boiling water bit. This effort pays off hugely for two reasons:
1.) You are adding real hops. Goo cans are hopped with "instant" hops.
2.) Because you don't get yeast with UN-hopped malt extracts you are forced to buy it. This means you are buying high quality yeast, and a yeast better suited to your recipe.