The noticable difference between User and Contact Object can be classified based on its Attributes.

Lets say, that User/Computer/Group – Fully Featured AD Objects.

Contacts/Printer – Compact AD Objects (Usefull in AD Search)

User Object:
Contains information about users like their first, middle and last names, login credentials etc. For example say Aroma, Paul and Alex areemployees of an organization, then a user object can be created for each one of them, This object will store various properties of the user like their name, the manager they report to, their sub-ordinates etc.

Contact Object: (Usefull in AD Search,eventhough they have no access to AD)
Contains contact info about any person associated with the organization like a supplier’s telephone number, mail address etc. For example an organization might want to store details of peoplewho are not directly associated with the organization such as people who help in network maintenance or suppliers etc.These people usually do not need to be given access to AD; only their details need to be stored which in this case is their name and contact information, this can be done using a contact object.