Everything in OOP ( Object Oriented Programming ) is centered around objects.
Objects are constructs in which a state and some behaviour ( actions involving this states ) is combined.
Objects can represent concrete subjects ( for instance persons ) or more abstract things ( for instance reservations ).
Every object is an instance of a certain class ( in case of "single classification" ).
Classes describe objects by stating which information ( state ) objects of this type can contain and which behaviour objects of this type have. In a class you describe how a representation of a concept is made, this is what is often referred to as the abstraction process.