To understand the security feature of SQL Server you need to understand the levels of security. So what happens at windows level, you need to user and password. As soon as you are inside windows you need to enter inside instance and for that you need a login. After entering into an instance you need to have users for each database. There will be mapping of users and login. So the main points are: To enter into an instance we need a login. To enter into database we need a user. Users and logins must be mapped. When a machine is on a network, it can be: Workgroup or Domain Workgroup is a standalone machine which may or may not be connected to a network and all the machines will be independent of each other while domain is a centralised machine where Active Domain(AD) is installed therefore it is called a Domain Controller(DC). So it is not needed in this case to go on each machine separately. But while installing windows...