Look UP relation ship in salesforce

1.You can define a lookup relationship between
any two objects, standard or custom.

2.When a parent object is being deleted,
you can configure a child object to either
clear the parent record value in the child
record or prevent deletion of the parent
record.

3.The child record may not require a
related parent record.

4.Each child record has an owner and is
not related to the parent record.

5. There is no security or inheritance
between related parent and child records.

6.To relate an object to other objects,
there is no condition on the number of records.

7.You cann't create the roll-up summary
field in the lookup relationship.

8.Does not support cross-object workflow.