Authentication vs. Authorization
Authentication is the process of establishing a known identity for the user, whereby ‘identity’ is defined in the context of the application. This may be done with a username/password combination, a cookie, a pre-defined IP or some other mechanism. After authentication the system believes that it knows who the user is.
Authorization is the process of determining, whether a given user is allowed access to a given resource or not. In most cases, in order to be able to authorize a user (i.e. allow access to some part of the system) the users identity must already have been established, i.e. he/she must have been authenticated. Without prior authentication the authorization would have to be very crude, e.g. “allow access for all users” or “allow access for noone”. Only after authentication will it be possible to, e.g., “allow access to the statistics resource for _admins_, but not for regular members”.
Authentication and authorization may happen at the same time, e.g. when everyone who can properly be authenticated is also allowed access (which is often a very simple and somewhat implicit authorization logic). In other cases the system might have one mechanism for authentication (e.g. establishing user identity via an LDAP lookup) and another one for authorization (e.g. a database lookup for retrieving user access rights).