Outline:

Three main aspects (stop, kør, kør igennem)
Entry signals and how they change the aspects
Exit signals
Signals inside a station (SI, PU, SU)
Left hand running and vekselspor

Shunting and dwarf signals

When signals don't work properly (forbirangering, stop og ryk)

Signs and boards

ATC and how it builds on top of signalling

Radio controlled traffic (i.e. stations with no exit signal)

To be up-to-date for signals around the year 2022

Danish signals are in principle extremely simple. There are three aspects: Stop (one red), proceed (one green), and proceed expect proceed (two green). Everything else is built on top of these simple aspects. To the right the three aspects can be seen on a three-lantern signal.

Note how this system is fail-to-safe, since upon the failure of one green light, the signal is more restrictive.

For different uses there are different signals. The most basic signals are two light signals, which can show stop (one red) and proceed (one green). Three light signals are shown to the right.

Automatic Block Signal (DK: Automatisk Mellembloksignal) is the simplest signal there is. It is found on the lines between stations, dividing the lines into smaller segments allowing for more trains to run between the stations at the same time. These signals cannot be controlled directly, they are fully automatic. AM signals showing a permissive aspect (proceed and proceed expect proceed) are dimmed unless there is a train approaching the signal. Restrictive aspects (stop) are never dimmed.

For more information on how AM signals are used, together with examples, see the AM signal main article

A station must always have at least one I signal. The I signal marks the beginning of the station and is passed as the train enters the station. The I signal can carry much more information than an AM signal, but the principles are the same.

In addition to showing whether the track ahead is clear or not, an I signal may also display the speed allowed to pass the signal. This can either be with a speed board, or with a light display. At some stations, there is a diverging line going somewhere else in one way or another. This is marked by a diverging arm on the I signal. This arm is used when the path is set to the diverging line, but has no direct influence on speed. Speed is still governed by the speed board or the light display.

For more information on how I signals are used, together with examples, see the I signal main article

The simplest stations may only have an entry signal (I signal). For any train leaving the station, permission has to be granted via radio. This used to be the case on the railway line from Struer to Thisted, and is the case on the line from Ribe to Tønder.
In more advanced stations movement authority from the platforms is given with a PU signal. This signal was originally designed for the ferry stations in Korsør and Nyborg to avoid having trains stop at the platform when they didn't need to. This has since evolved into use also outside platforms, but the name has stayed (PU = platform signal, DK: PerronUdkørselssignal).
The PU signal is a combined signal in the sense that in addition to train movement authority it also can show shunting permissions. For more information on that, see PU signal.
When departing the platform by authority of a PU signal, all following PU signals are expected to show the same. They repeat themselves and can sometimes be found very close to each other - for example along curved platforms. The pre-signalling indicated thus applies to the next signal not a PU signal, usually a U signal (station exit signal).

For more details and use cases, see the PU signal main article.

Some stations are quite large, and thus warrant non-repeating signals to divide the station into smaller sections. These signals are called SI and SU signals for the entry and exit part of the path respectively. SI signals function almost like an I signal with some exceptions.
SU signals are like U signals (exit signals), but with the additional possibility of showing speed limit with a board.

For more details and use cases, see the SI signal and SU signal main articles.

For train movements within a station, shunting signals provide movement permission. Not all stations have the capability to show all shunting signal aspects, it depends on the version of the signal control system and what features are implemented.

For more details and use cases, see the Shunting Signals main article.

In Denmark the responsability for warning cars is on the railway. Thus, crossings are either secured by lights or on lower speed lines unsecured but warning with train horn. When a road crossing is secured with lights and bells, the train driver is informed of the state within braking distance from the crossing. If the crossing is not secured (broken lights, open barriers etc), the train driver must try to stop before the crossing.

Level crossings between platforms within a station and some road crossings are tied to the main signals. Thus they cannot show a permissive aspect while the crossing is not secured.

For more details and use cases, see the Level Crossings main article.