What are the difference between them?
they are not different architectures as the first person tried to make out.
intel works on a tick tock cycle. this means they release a new architecture then they release and improved version of that architecture then a new architecture and so on.
sandy bridge was an old architecture ivy bridge and haswell are very similar architectures.
the performance jump from sandy bridge to ivy bridge is fairly large because of this i wouldnt bother with sandy bridge.
what happened between ivy bridge and haswell is very interesting though. intel knows they have the most powerful processors so rather than increasing performance (the performance increase of equivalent haswell chips is barly 10% from the ivy bridge processors) they increased efficiency.
for people that want the best procesors this is both good and bad news but at the same time it is very good news for tablet PCs.
you see the is a never ending struggle to increase the power efficiency of processors, making them more powerful is a piece of cake however more powerful processors = more heat = more expensive cooling.
because of this haswell isnt anything special for people that want more performance right now, unless they are particularly worried about their electricity bill, but in the future it is a very good thing. This huge leap in power efficiency (we are talking laptops with haswell proessrs jumping from a 5 hour battery life to close to 12 hours) means that less heat is produced.
now intel has until the next processor release date (12 - 18months) to do nothing but add performance to these chips a the power requirements are already so low per unit area of silicon used.
for laptops and tablets what does haswell mean. well its the first 22nm fabrication process used, this is what aids to the power efficiency. it also means more power in smaller devices, having the components so small means you can fit more per unit area meaning ultrabooks and tablets can have more powerful processors while maintainins their size and battery life.
it also means the possibility of tablets running things like the i3 without much cooling atall, it think i saw that the i3 4000 series for mobile/ultrabook would happily run at 22.5wattts, this can almost run with passive cooling. this means thinner and more powerful tablets and ultrabooks. at the same time as reduced price as companies dont have to invest loads in R&D for cooling
i think that sums up all the differences :)