What is the difference between semantics and pragmatics?

Semantics is most easily explained as representing the 'dictionary meaning' of something - what you'd interpret as the literal meaning of a sentence, for example, is being analysed with the theories of semantics. Something like 'Oh, it's a bit cold in here, isn't it?' would just be interpreted as a comment about the temperature if you're only taking semantics into account. Pragmatics, on the other hand, considers 'underlying' meanings that are deduced by the speaker and listener's shared knowledge (for example about social norms). Pragmatically, you could take 'Oh, it's a bit cold in here' as a hint that you should close the window. You might only get this if you'd for example argued about whether to open the window in the first place - only you and the speaker know this and so the pragmatic interpretation is dependent on context!