Theory:

The United States and Great Britain are two countries separated by a common language — George Bernard Shaw.
 
English, as a language, is spoken all around the world. But do you think everyone speaks the same English?

The two most widely accepted forms of English are:
 
1. English from Great Britain (British English)
2. English from the U.S.A (American English)
 
Differences:

The Great Britain is essentially where English was born. As a language which branched out from Latin, British English borrows a lot of words from its sister languages such as French, German, Portuguese, Italian, Spanish and other European languages. For this very reason, British English has spellings in which the letters look like they have been added unnecessarily. This is because British English takes words directly from other languages and uses them; it adds to the beauty of the language.
 
American English, on the other hand, is highly focussed on how exactly a word is spelt and follows direct pronunciation of its spellings. This is why though American English is different in many ways from British English, it is found to be more direct and focusses its attention more on function rather than beauty.

This does not mean that American English is less beautiful compared to British English because some of the greatest English poets, such as Robert Frost, Maya Angelou and Emily Dickinson, are Americans.

Some of the spelling differences are listed below:
 
S.No
British
American
1
FlatApartment
2
LiftElevator
3
ColourColor
4
HumourHumor
5
OrganiseOrganize
6
CenterCentre
7
PetrolGasoline
8
BiscuitsCookies
9
HolidayVacation
10
NeighbourNeighbor
11
AeroplaneAirplane
12
ProgrammeProgram
13
CentreCenter
14
OffenceOffense
15
ChipsFrench fries