American English and UK English
It is often said that Britain and the United States are two countries divided by a common language. More recently, the linguist Robert Burchfield suggested that US and UK English were separating so fast that Americans and British people would not be able to communicate by 2050. More recently still, a member of the House of Lords described US English as the most hideous language on the face of the earth. But, like it or not, US English is the most influential variety of English internationally, and 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.
Strictly speaking, using US spellings in the UK is wrong, except when they have already been accepted into UK English. 'Jail' (not gaol) is one example, or 'today' (not to-day). In both those cases, and lots of others, the US spelling is now the usual one. But other US spellings haven't caught on, yet. 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.