The classes are the base of the Object Oriented Programming ( OOP ). Actually, they have a similar behavior to the structs in the C/C++ languages, and are very similar to the ones in other OOP languages; por example, Java.
Since the classes are the base of the C# language, the first step is to learn what the are and how they work. At this point, let's only say that a class is a special type defined by the user, which encapsulates variables ( now called Fields ) and functions ( now called Methods ), designed to interact the first ones with the second ones, as they would do in a real world.
Let's try to make it easier to understand. Imagine you are at home, and your girlfriend says she need some spices to finish the food. So, we can say this in a OOP paradigm, that the object of the class Human called Girlfriend is interacting with another object of the type Human, called Boy, asking him for a result, calling the PlaseBuySpecies function/method, passing as parameters the names of the species she needs. Yeah, this can be weird, but trust me, we are in the right way.
Then, you go to the supermaket and you see you can't find the required species, sho you go with and pay for help to the attendant. You, you, the variable Boy are asking to the variables Attendant of type Human for the localization of some objects, passing their names as parameters. You see, you don't need to know how she does it; you only will be expecting for the results.
Finally, you go home and give to your girlfriend the species, finishing the request to the PlaseBuySpecies method, returning the species themselves. So, now - I hope - you understand this situation, you will bea ready for the next section.