Elements of sketch.

I try to understand sketch from examples.
From the very beginning I have:
#include "xxx.h" // this line includes library
next line is like that:
xxx XXX;

Where xxx is the name of the library but what is this?
Is it a declaration? of what?
Is it function?
What exactly this statement means?

In void loop() section XXX is called to read data.

Thx in advance

That line is just like a variable declaration. Where you think you see the name of the library, you actually see the name of a class that library defines. Then where you have XXX is the name of the object of that class you are creating.

Google Object Oriented Programming for more than you ever thought you wanted to know.

Google C++ classes for more information on this particular syntax.

Than you Delta_G.
That means this class must be defined in the library.
In my case, xxx it is the class from the library, and XXX is my naming for that class by what object is called. This is how I understand it now.
Thx again