Another important difference between the two is spelling rules. American English tends to use "-ize", "-ze" and "-ization", whereas British English uses "-ise", "-se" and "-isation". For example, US: emphasize, analyze, organization, UK: emphasise, analyse, organisation. British English also includes a "u" after "o" in certain words, whereas American English does not. For example, US: color, flavor, UK: colour, flavour. With words derived from Latin and French which end in "-er"/"-re", American spelling rules use the "-er" ending and British spelling rules use the "-re" ending. For example, US: meter, center, UK: metre, centre. With words containing "ae" and "oe" in British English (usually medical vocabulary), it is simply written as an "e" in American English. For example, US: gynecology, estrogen, UK: gynaecology, oestrogen.

As Britta mentioned, some of the vocabulary is also different. Here are some examples:

US: sidewalk, UK: pavement

US: elevator, UK: lift

In some cases, the meanings have changed so that the same word is used to mean different things entirely. This can even lead to misunderstandings between Americans and Brits. For example:

US: pants, UK: trousers

US: underwear (for guys), UK: pants

US: fries, UK: chips

US: chips, UK: crisps

Hello! Allow me to make corrections for you:

What are the main differences between British English and American English?
Which one do you like the most?

To answer your question:

The main difference is the vocabulary. I know the British use different slang, and also still use some "old-english" words. Then of course there is the obvious difference of dialects (accents).

British English and American English

Which are the main differences between British English and American English?
Which one do you like the most?

Same difference between Spanish in Mexico, other Latin countries and in Spain.  You can understand 90% except when it comes to certain phrases.  It may mean one thing here and another there.