PHP is an object-oriented programming language, which means that you can create objects, which can contain variables and functions.
When talking about objects, you refer to variables belonging to these objects as properties (or attributes or fields), and functions are called methods.
These objects are essential when dealing with PHP, as almost everything is an object: for example, functions or arrays are objects, too!
And this shows why we use objects: we can bundle our functions and data in one place, we can create objects easily using classes (object constructors), so we can create lots of instances (objects, which have been constructed via a class), which contain mostly the same data, except some little nuances.
Then the stylesheet adds some color to the result. :-)