small tasks were put in to functions via the top down or bottom up approach.
These functions were then called in a specific sequence to form a program.
Generally, programs were entirely written in one file.
With object oriented programming...
You define the classes, which are the nouns, and the methods that act on
those classes, which are the verbs.
Makes debugging easier, with encapsulation problems can be systematically
found.
Makes group programming easier as well.
You can make multiple instances of a class for re-use.
There is a lot more here than we can cover...
You could take an entire course on object design with UML.
You could take many courses on object design alone!