Access control is simply allowing the right people into an area at the right time,
or
"Saying who can go where when"

Some examples
Security vs Access Control
Who Where When

Some examples
The public is allowed in the front door during office hours. The door will automatically unlock at 9AM and lock again at 5PM. Anyone outside the building can get inside during these hours.

On the same door, it is necessary to allow people to enter outside these times. They will be issued with a credential (eg a card, key, or other identification means). They will then be assigned times in which they can enter. Some people may only have extended hours (eg 6AM to 11PM Monday to Friday), while others may have 24 hour access.

It is possible to restrict access through doors, driveways, and lifts.

Security vs Access Control
Access control often plays a key role in a building's security system, but in one key aspect the two are directly opposed:

Security systems are designed to keep the wrong people out, access control systems are designed to let the right people in.

For example, security includes bars, cement, guards, dogs, electric fences, alarms, detectors and the like. They are all aimed at detecting an intrusion and responding to it.

An access control system is the "loop hole" in the security system that allows the right people to enter and exit. The better the access control, the less wrong people will be able to enter. PIN numbers are easy to pass around and provide a minimum amount of security at a cheap price. Biometrics (eg retina scanners) provides a high level of security at a very expensive price.

The best solution weighs the risk and cost of intrusion versus the cost of the security and access control systems.

Who
All users of the system will be assigned a credential. This credential may identify an individual (eg a key or card) or a group of people (eg a PIN).

The better credential are those that can not be copied. They should also be easy to use and convenient to carry.

Lost credentials can be immediately removed from the access providing immediate security. Unassigned keys will also be denied access.

Where
Users will be typically assigned to a group. This simplifies the administration of a large number of like users.

The group will have what doors and floors members will be able to access. It is possible to have different areas covered by different groups. For example, a group could be defined by tenant, department, or even an individual.

It is also common to have slight variations in groups to provide for the option of parking and other benefits.

When
Time zones provide the ability to specify what times are secure and unsecure. Time zones will then be combined with doors in groups to specify what times users have access.

It is possible to provide different times to different doors within one group. For example users could have 24 hour access into the building but only office hour access to certain floors.

Doors can also be allocated time zones that will automatically lock and unlock them. This provides public (ie non credential holder) access.

Start and end dates allow keys to be automatically added and removed from the system. This allows keys to be sent to contractors, guests etc and they will not have access until they are officially allowed in.