C# provides built-in data types, like string: each instance of the string type has its own values and functionality.

string phrase = "zoinks!";
Console.WriteLine(phrase.Length);
Console.WriteLine(phrase.IndexOf("k"));

In this case phrase is an instance of the string type. Every string has a Length property and IndexOf() method, but the resulting values are different for each instance.

A class represents a custom data type. In C#, the class defines the kinds of information and methods included in a custom type.

You can then make instances of that class (above, phrase was an instance of string). There may be many instances of the same class, all with unique values.

To begin defining a class, C# uses this structure:

class Forest {
}

The code for a class is usually put into a file of its own, named with the name of the class. In this case it’s Forest.cs. This keeps our code organized and easy to debug.

In other parts of code, like Main() in Program.cs, we can use the class. We make instances, or objects, of the Forest class with the new keyword:

Forest f = new Forest();

We could say f is an instance of the Forest class, or f is of type Forest.

The process of creating an instance is called instantiation. Today we instantiate a class; yesterday they instantiated a class, and so on.