Mohammed,
You can find the answer to your question in the Glossary of the PMBOK Guide. Here is my explanation:
A Milestone is a "significant point or event in a project, program or portfolio". A Milestone is usually displayed on a project SCHEDULE.
An Objective on the other hand is "something toward which work is to be directed, a strategic position to be attained, a purpose to be achieved, a result to be obtained, a product to be produced or a service to be performed." Another word for objective is "Goal". An objective is usually included in the Project Charter.