Need basic info

Hello I wanted to learn basic scripting so can u just tell me a little about this things

1- what does Public means and what is its use?
2-what does Private means and what is its use?
3-what does callback means and what is its use?
4-what does Strings means and what is its use?

And if there is anything else u can tell pls tell me cause I really want to Learn scripting and programming