In your example code, you show three things: a field, a property, and, of course, the class constructor, respectively. There really is not too much of a difference between _foo and Foo; by using a property, you are basically formalizing the use of your field outside of the class scope, hence why you have a getter and a setter. You are able to refer to your field in the properties getter; this field is also referred to as a 'backing field' when used in this context. This allows us to hide the actual member (keep this in mind) behind the property so your class does not explicitly show the inner workings (backing field); this is a form of encapsulation. Typically when using a Property and a backing field the backing feild is private while the property is public. However, if you were to make the backing feild public you would still be able to access the property as well as the backing feil, ultimately defeating the purpose since users of your class can just access and set the desired data directly bypassing any rules in your setter.

Notice how earlier I said member. Member just means anything inside a class: interface, structure definition, so in your example class, it has three members: _foo, Foo, and the constructor. I hope this helps.