Hi Mohammed,

•  Lookup links two objects together.
• Link an object with itself (with the exception of the user object; see Hierarchical) .

1)In lookup relationship field value is not mandatory
2) Here parent record is deleted automatically child records are not deleted
3) An object is allowed only 25  relationship fields
4) Here parent rules and child rules are may be same or not.
5)If we cannot give a value to the lookup field then we can't  directly conert lookup relationship to master-detail relationship here first we need to give a value to the lookup field.
6) If we give a value to lookup relationship field we can change that value whenever we required.
Lookup Types:
External lookup
An external lookup relationship links a child standard, custom, or external object to a parent external object. When you create an external lookup relationship field, the standard External ID field on the parent external object is matched against the values of the child’s external lookup relationship field. External object field values come from an external data source.
Indirect lookup
An indirect lookup relationship links a child external object to a parent standard or custom object. When you create an indirect lookup relationship field on an external object, you specify the parent object field and the child object field to match and associate records in the relationship. Specifically, you select a custom unique, external ID field on the parent object to match against the child’s indirect lookup relationship field, whose values come from an external data source.
Hierarchical
A special lookup relationship available for only the user object. It lets users use a lookup field to associate one user with another that does not directly or indirectly refer to itself. For example, you can create a custom hierarchical relationship field to store each user's direct manager.

Please refer the following link for more information about Lookup.
https://help.salesforce.com/apex/HTViewHelpDoc?id=overview_of_custom_object_relationships.htm&language=en (https://help.salesforce.com/apex/HTViewHelpDoc?id=overview_of_custom_object_relationships.htm&language=en)


Regards,
Jyothsna D