Explanation of where is best to code and what please

[eluser]Unknown[/eluser]
Hi All,

I am fairly new to CI and MVC, but would love if someone could give me a quick explanation of where I should be coding etc

What should I code in a library? Controller?, Model?, View?

I know models are for the database stuff? But I have seen a lot of this type in libraries also?

So what is the right thing to do?

I hope this makes sense.