American English and UK English
US spellings are becoming more common in all varieties of English, including UK English. The influence is especially strong in scientific English, where a computer has a hard disk with a program on it, not a disc with a programme. But strictly speaking, using US spellings in the UK is wrong, except when they have already been accepted into UK English. In the UK a boat anchors in a harbour, not a harbor, and a joke is an example of humour, not humor. Here's a list of common words which are spelt differently in the UK and the US.