Abstract type
In programming languages, an abstract type is a type in a nominative type system that cannot be instantiated directly; by contrast, a concrete type can be instantiated directly. Instantiation of an abstract type can occur only indirectly, via a concrete subtype. An abstract type may provide no implementation, or an incomplete implementation. In some languages, abstract types with no implementation are known as protocols, interfaces, signatures, or class types. In class-based object-oriented programming, abstract types are implemented as abstract classes, and concrete types as concrete classes. In generic programming, the analogous notion is a concept, which similarly specifies syntax and semantics, but does not require a subtype relationship: two unrelated types may satisfy the same concept. Often, abstract types will have one or more implementations provided separately, for example, in the form of concrete subtypes that can be instantiated.
Class - In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state and implementations of behavior.
Concept - In generic programming, a concept is a description of supported operations on a type, including syntax and semantics. In this way, concepts are related to abstract types but concepts do not require a subtype relationship.
Type class - In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types.
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