Informal: kids, guy, awesome, a lot, etc.
Formal: children, man/boy, wonderful, many, etc.
Informal: can't, won't, shouldn't, etc.
Formal: can not, will not, should not, etc.
Here is an example of formal English that you read in books:
As the price of fours dollars was reasonable, I decided to make the purchase without further thought.
The same sentence would be said quite differently in informal English. Here's an actual example that you will hear from a young American:
It was, like, four bucks, so I was like "okay".
What should you learn?
According to a research, it is proved that if you learn Informal English! then it's very easy for you to learn Formal English without even learning it deeply. And as every body knows the important Formal English, then one should learn Informal English first. This is the English which is being spoken on the streets of America. ELP's lessons focus on both Formal and Informal English. By just listening, you can have command on both types of English.