All over the world there are countries where the native language is English.
You can look the list here
The English language is almost the same, If you understand English from London, You can understand the English of Australian native speaker too (and everyone speaks English).
There are a bit of differences in every type of English, like slang, accent and some words.
Example: In American a lot of people use "What's up", In England almost nobody does it. In American everyone says "color", in British "colour".
I can't explain every differences, You can search on Google
Difference between english and [other english language, like Scottish etc.] for get a lot of differences between English and other types of English.
I link you this article "What Are The Differences Between American And British English?"
But in the web you can found more thing about it!