AIS is short for Automatic Identification System. AIS is a VHF-based navigation and anti-collision tool making it possible to exchange information between ships. This information, AIS data, is furthermore collected in a shore-based AIS system operated by the Danish Maritime Authority. Thus, distinction is made between the AIS equipment on board ships and the shore-based AIS system when referring to AIS.

There are two types of AIS transponders on board ships, class A and class B. Large ships are required to carry a class A AIS transponder, which is a more expensive and more advanced model compared to class B.

Class A is required on board:

- All ships of more than 300 gross tonnage
- All passenger ships
- All fishing vessels with a length above 15 metres.

Small vessels that are not required to be fitted with AIS, such as recreational craft, can use a less expensive AIS class B station. The class B type does not transmit as often as the class A type, and large ships can choose not to show the AIS class B stations on their displays if it becomes impossible to get an overall view because of too many recreational craft.

There are three categories of AIS information:

- Identification of, for example, the ship's name, call sign or IMO number as well as its MMSI number
- Navigation related to, for example, the ship's position, course and speed
- Information about the ship's voyage, for example its destination, time of arrival and its actual draught.

AIS contributes to safety of navigation and has, in many ways, made it easier to navigate safely. However, AIS also has some limitations that it is important to take account of. If you trust your AIS data uncritically, it may be very risky.

You must also be aware that the information received by AIS originates from the ships' own instruments. There can be errors in the information inserted or technical defects that may give an erroneous impression of the ship's speed or position.

When installing AIS on board a vessel, it is therefore important to use a qualified technician and to insert all information correctly. Use only type-approved CE-marked equipment.

AIS and radar are two very different systems.

The radar shows all objects around it, but can only show the position, and perhaps a calculated course and the speed of the objects. In addition, the radar does not have as long a range as AIS.

AIS provides information about the name, course and speed of near-by ships – and AIS is better at seeing ships that are far away or "in the shade". But AIS sees only ships fitted with AIS and can, therefore, not replace a radar. The radar is the primary instrument for collision prevention.

The Danish Maritime Authority has placed a number of AIS stations along the Danish coasts, making it possible to also exchange information between the ships and the shore-based AIS stations.

The information from these shore-based AIS stations makes it possible to present a here-and-now picture of the ship traffic in our waters.

Simultaneously, the system will be storing AIS data which can be used to reconstruct and analyse accidents and incidents, such as collisions and groundings. Major extracts of historical AIS data are used for many purposes, for example statistical analyses of traffic patterns in Danish waters.​

Especially important aids to navigation can be fitted with an AIS transponders. This is an especially arranged AIS, which transmits data messages referred to as "Aids to Navigation reports", but they can also have been transmitted via the shore-based AIS system as virtual AIS aids to navigation. In general, virtual AIS aids to navigation are used for sudden dangers, such as the marking of a wreck that presents a danger to traffic. It requires a permit from the Danish Maritime Authority to establish an AIS aids to navigation similar to an ordinary aids to navigation.