What is the difference between a User, Contact and an Employee?

The answer to this question is subjective hence open to manipulation based on personal perception.  Here is my version of the answer of this question. Answer:

Employee: An employee is user who can login to the application and has an employee profile associated to it. He is also present as the Contact in Application. So, we can say that every employee is also a contact.

User: In application like eCommunication, ePharma you can provide anybody with ability to register and use application with limited access that is a User in Siebel. A user can also be a contact but a User is not an Employee.

Contact: Contact is a person who has least of privileges when it comes to application. A contact is not a part of organisation but is usually associated with Accounts (Customers) as a touch point to reach customer. This is a basic detail of all three.

Main point of differences that I can think about these are:

Application use: A Contact has least privileges and he cannot login to application where as an user and employee can login and use Siebel Application.

Screen: The Screen to access Contact records is Contacts. Employee and User share the same screen that Administration – User but they have different view.

Table: Base table for Contacts is S_CONTACT where as User and Employee are based in S_USER. In pervious version of Siebel there was table called S_EMPLOYEE which was used but in Siebel 7.8 that table in inactive. General information of an Employee and Users such as first name, last name etc are stored in S_CONTACT and other information is stored in S_USER.

Employee Flag: Employee Flag is true for Employee but false for an User and Contact.

Thats my answer if you think I am wrong some where or I have missed something (I am sure there is lot more than this) please contribute and I will update the post with the points that I find relevant so that we can have a pretty conclusive answer to this question.