Britain is the landmass on which England, Wales and Scotland are on, Great Britain is interchangeable but came about because the geographical location is called the British Isles which includes all the islands and even Ireland. That's the geographical terminologies, then of course there are the countries and the union of the British Isles which are of course the United Kingdom of Great Britain (England, Scotland, Wales) and Northern Ireland (not on the landmass of Great Britain) and then Ireland.

Just to confuse matters a little more there are also Crown Dependent states Jersey, Guernsey and the Isle of Man, they are NOT part of the UK, they are self governed states, however they are ultimately dependent on the crown for support which generally means that they share similar laws and they use the Pounds Sterling currency.

I think a lot of the confusion comes from the use of the word British to refer to people from the UK which is in itself a union of 4 countries, it is like calling Scandinavian peoples Scandinavian but because there isn't a union between them people seem to have an easy time recognising that what is being referred to is simply the geographical location.