In Australian English, they are saying "mate", not mike. Mate is a friendly term. As far as dialects go, there are so many even within each country it's hard to say. I'm a native English speaker from America, but I can fully understand people from UK, Australia, Ireland, Scotland, etc. In America, there are several different dialects. I'm a Texan and have been all my life. Most of my family has a distinctive accent, but I, for some reason, do not. I'm pretty good at imitating accents though. :-)

American English is the only one worth knowing :)

"Can you explain the differences between English dialects, such as UK English and US English? What is the difference between these two dialects? I've also heard that the English used in Australia is different as well - quite a bit, such as using 'mate' in daily speech. For example, 'How are you today mate?' " Please pay attention to how you write your questions - this is the corrected version of what you wrote.

There's no such thing as Australian English, only British English with an Australian accent. Some slang is different, but it's not used nearly as often as you think. Here are some differences between American and British English: http://www.italki.com/question/199751