Welcome to JavaRanch.
A lot of big questions, actually. I shall try to answer a few.
The real idea behind object-oriented (OO) programming is that you create software "objects" which have data (called fields in
Java) and behaviour (called methods in Java).
You can have a few objects or you can have as many as you can fit onto your computer's memory (I once tried and go to 6000000, but I didn't examine them individually!) Objects can communicate with one another; they send "messages" which tell other objects to do something. In Java we call those messages method invocations.
A class represents an abstraction; a Car class might have speed and colour, but you can't tell what colour the Car class is, nor what speed it is travelling at. [This is different from an abstract class; forget abstract classes for the moment.]
An object is also called an instance of its class; you can have a Car object which records it is "red" and travelling at 60mph.
The class encapsulates all that, colour, speed, going faster, stopping, etc. It includes code to do all those things. It is a good idea to keep the data hidden so other objects can only reach them
via methods, and to hide the way things are worked out (data hiding).
I hope that helps as a start, and I hope other people will have their own answers to add.