Ole-Johan Dahl and Kristen Nygaard presented the design for Simula 67 in 1967. This language allowed specifying objects belonging to different classes while sharing common properties.
Single inheritance occurs when a class acquires features from one superclass without exception for constructors or destructors. Multiple inheritance allows one class to inherit features from more than one parent class simultaneously.
Subtyping establishes an is-a relationship whereas inheritance only reuses implementation establishing a syntactic relationship. Inheritance does not ensure behavioral subtyping even if it commonly used for subtype relationships.
The colon indicates that the subclass inherits from the superclass in C++ syntax with optional private or public visibility modifiers. Default visibility remains private when no modifier appears in the declaration.
Allen Holub identified the fragile base class problem as introducing unnecessary coupling through implementation inheritance. Complex inheritance within insufficiently mature designs leads to the yo-yo problem creating many very thin layers of code.