Abstraction

Imagine a fraction class:

class fraction:
    int denominator
    int numerator

Now two objects of that:

fraction(obj1): denominator=-1 numerator=-1
fraction(obj2): denominator=1  numerator=1

Both objects have the value 1: (1/1) == (-1)/(-1). You wouldn't expect they behave any different to the outside. That's abstraction. You abstract the data your object holds into a logical view, even tho behind the scenes, there are other things. Theoretically, you have got a equivalence relation, with different equivalence groups:

[1]=(1, 1), (-1, -1), (5, 5), ...
[2]=(2, 4), (-2, -4), ...
...

And there is a abstraction function that abstracts the internal details to the outside:

f((1, 1)) = [1]
f((-1, -1)) = [1]

It maps from concrete values to the abstract values of an object. You do that by writing for example a constructor mapping (-1, -1) to (1, 1) and by writing a equals function for your class.