Inheritance is the mechanism which allow to generalize / specialize classes into hierarchy, the base class being the most generic, the classes inheriting from this base class adding some specialization.
Example:
Polymorphism is the ability to use an object without knowing exactly is type, just by using the base class as a type and being able to call method (virtual method) for it. Having a virtual method StartEngine() on Vehicle, and having this method redefined for all the other class will allow to call that method on a pointer on Vehicle, and the corresponding method in the correct class will be called thanks to polymorphism.