American English and British English differ in three main ways.

First of all, the same words are sometimes spelled differently.
British words with an “ou,” American words with an “o.”
favourite vs. favorite
colour vs. color
honour vs. honor
neighbour vs. neighbor
behaviour vs. behavior

British words spelled with an “re,” American words with an “er.”
theatre vs. theater
centre vs. center
metre vs. meter
kilometre vs. kilometer

British words spelled with an “s,” American words with a “z.”
analyse vs. analyze
realise vs. realize
criticise vs. criticize
apologise vs. apologize
organise vs. organize

British words spelled with a “c,” American words with an “s.”
practice vs. practise
licence vs. license
defence vs. defense

British words spelled with “ll,” American words with one “l.”
travelled vs. traveled
modelling vs. modeling
fuelled vs. fueled
signalling vs. signaling
counselling vs. counseling

Second, the same words sometimes have very different meanings.
British vs. American
chips vs. french fries, not potato chips
trousers vs. pants
pants vs. underwear/panties
football vs. soccer, not American football

Lastly, sometimes we have different words for the same things.
British vs. American
lorry vs. truck
loo vs. bathroom
post vs. mail
holiday vs. vacation
motorway vs. highway
rubber vs. eraser (a rubber is a condom in American English!)
pavement vs. sidewalk

British vs. American expressions
have a bath vs. take a bath
at the weekend vs. on the weekend
bring up a child vs. raise a child
have you got vs. do you have (for possession)
British use “shall” instead of “will” to talk about the future in the first person.
British use “shall” to make an offer, Americans use “should” or “would.”
Americans do not use shall because if they did they would get their asses kicked..