Why do you omit "public" infront of the class header?

Question

I am currently using a book (along with this tutorial) to learn Java. And my book tells me to do:

public class HelloWorld
{
//lines of codes
}

instead of

class HelloWorld
{
//lines of codes
}

like this tutorial teaches.

Is this a matter of style? (Does it differ between programmers?)
Or is it a matter of syntax that is not necessarily wrong to do so, and it can be done?

Please forgive my misuse of terminology, if I misused any.

Thank you!

Answer 1

Public means that the class or method can be accessed from another package. If you leave it out only elements of the same package can call it.

Answer 2

main()
is global function that must require anything outside class
if private that mean your class can be accessed by only inside it

Well I might be wrong :(
cus I don't deeply in class/constructor