static indicates the class can only have static members and you cannot create an instance of it. This is used for stateless functionality (for example a type that just defines extension methods, or utility methods). You can also declare a member static on a non-static class. This allows you to attach functionality to a type without having to instantiate it.
Here’s more detail on using static members and classes.
abstracts define the basic structure and functionality shared by all derivative types, but cannot be used by themselves. Think of them as, I suppose, a blue print and a contract. This is a core concept for OOP.
Here’s more detail on using abstracts.