There's an old saying that America and Britain are two countries divided by the same language. American English is different from British English, but why? The reason is partly cultural and partly historical.
When British settlers arrived in America, they brought the language of Elizabethan England with them. Words such as 'fall' for 'autumn', 'trash' for 'rubbish' and a slew of other American words are old English words that the British no longer use.
Over time, the language of the settlers evolved as they interacted with the native Indians and others who lived in America. Foreign words such as 'barbecue' (native Indian), 'ranch' (Spanish), 'chowder' (French), 'okay' (African), 'cookie' (Dutch), and many more were added to the language that we now know as American English. With so many cultural influences on the language, it's amazing that the British can still understand American English.
However, there are still enough differences to create confusion over something as simple as ordering tea. In America if you order tea, you'll get ice tea unless you request hot tea.
Some words used in both American and British English have different meanings. For example, in the US 'pants' are trousers, not underpants, people walk on the 'sidewalk', not the pavement, and they eat 'potato chips' not crisps.
Americans say things differently, too. The so-called American 'twang' refers to standard pronunciation in the US - the way most, but not all, Americans pronounce their words.
Americans tend to slide over the letter t or pronounce it as d, so mountain sounds like 'moun'in', and daughter is pronounced 'dawder'. But most Americans do pronounce the letter r, unlike the English who say 'cah' for car.