Inheritance can be thought of as an is-a relationship. For example, a dictionary is a book.
class Book end class Dictionary < Book end
Association can be thought of as a has-a relationship. For example, a library has books, so there is an associative relationship between objects of class
Library and objects of class
class Library def initialize @book = Book.new end end class Book end
See also Collaborator Objects in What is Object Oriented Programming.