Tipically on a modern computer system there are two types of account: computer and user.
User Account
A user’s account allows a user to authenticate to system services and be granted authorization to access them; however, authentication does not imply authorization.
User accounts are the primary means of access for real people to the system, and these accounts insulate the user and the environment, preventing the users from damaging the system or other users, and allowing users to customize their environment without affecting others.
Every person accessing your system should have a unique user account. This allows you to find out who is doing what, prevent people from clobbering each others' settings or reading each others' mail, and so forth.
Account (Computer Science)
Each user can set up their own environment to accommodate their use of the system, by using alternate shells, editors, key bindings, and language.
Computer systems are divided into two groups based on what kind of users they have:
User accounts on multi-user systems typically include a home directory, in which to store files pertaining exclusively to that user's activities, which is protected from access by other users (though a system administrator may have access). User accounts often contain a public user profile, which contains basic information provided by the account's owner.
To log in to an account, a user is typically required to authenticate oneself with a password or other credentials for the purposes of accounting, security, logging, and resource management.
Computer Account
On an Active Directory domain, in a Windows environment, user accounts and computer accounts represent a physical entity such as a computer or person. User accounts and computer accounts (as well as groups) are also referred to as security principals. Security principals are directory objects that are automatically assigned security IDs (SIDs), which can be used to access domain resources.
Every computer running Windows NT, Windows 2000, Windows XP and above, or a server running Windows Server 2003, or 2008 or 2012 ,that joins a domain has a computer account. Similar to user accounts, computer accounts provide a means for authenticating and auditing computer access to the network and to domain resources.
Why these?
These account types usually are needed for the following reasons: