Is there any big differences with the Iphone 4 and the 4s?

I know that the Iphone 4s has siri, but is there really any other main differences? And i will be buying this off the apple website and i'm currently with o2, how would i go about continuing with this particular service provider- i'm also pretty confused about what 'unlocked iphone' means? Haha Thanks.

Both iPhones can browse the internet and read the answers given to the last 100 or so people who already asked about the difference between the two.

Unlocked means that its not stuck with just one carrier.