There are different relationships between objects.

IS-A

extends or implements means IS-A relationship.

For example, Apple extends Fruit means apple is a fruit, but fruit is not apple.

HAS-A

HAS-A relationship contains Assocation, Aggregation and Composition.

This relationship stands for one-to-one, one-to-many or many-to-many.

Dependency

A dependency relationship indicates that changes to one object can cause changes in another object.