An access control system is used in both the field of physical security and the field of information security. It's generally any method of controlling who does or doesn't get access to something. Historically speaking, using metal locks and keys are the most basic way of securing things and preventing unauthorized access to them.
In the digital realm, login credentials are another access control system. Basic systems use a username and password combination. More complicated systems might also involve two-step authentication using a verification email or text message to a smartphone. Security checks might also involve manual clicks on quiz questions or selecting certain pictures that match a certain description.
One common kind of access control system is that of physical security keeping control over who is able to enter a specific location. Subway systems in many major cities use turnstile systems, and expressways that require toll to be paid would have toll booths where someone must stop and pay the fare in order to access that stretch of highway. Newer technology might allow people to pass through with electromagnetic or radio passes. Sometimes, facial recognition software or advanced biometrics are used to identify who is allowed to come into a place.
Border crossings are access control systems used by nations in order to manage who comes in and out of their territory. People looking to enter a country might have to prove their identity, where they are from, why they are visiting, and that they are in compliance with the regulations, rules, and laws of the nation that they are trying to enter. Entry can be denied to those who aren't supposed to be coming in or don't have permission to come into a country or territory. On a smaller scale, physical fencing around a property can be an access control system of its own, simply preventing someone from easily getting onto a property.
Access control systems are often used in corporate headquarters where there might be many workstations where most employees could access the sensitive information and data of clients. Call centers are commonly known to use this too. Some campuses might have triple-layer access control systems, where employees and visitors are first verified at the vehicle gate or parking entrance to the entire facility. They would go through a second check possibly using badges or swipe cards to enter specific buildings and departments. A third-level access control system would be their login and password to access an organization's intranet.
There are many reasons why companies, establishments, and organizations would use access to control systems. For starters, keeping trespassers and unverified individuals out is a security matter when protecting corporate assets, private information, and any proprietary property. Even people who mistakenly wander in thinking the office is something else entirely can pose a security risk to the organization.
Secondly, access control systems are in place to prevent crime and corporate espionage. In a digital and interconnected age where hackers on one side of the world can shut down an oil pipeline on the other side of the planet, the need for robust and high-level security is more essential than ever. While pipeline stations need to have physical access control systems to prevent sabotage on a material level, digital access control systems should be in place to prevent hacking attempts.
Third, organizations need access control systems just to manage their own teams of people. Former and recently terminated employees need to have their property access revoked immediately. Even among current and active staff, access to certain files and physical resources needs to be restricted to those that are trusted and responsible.