die Wache
guard
A person or group of people (e.g., soldiers, police officers) whose job is to guard and protect a place or someone.
Die Wache am Eingang kontrollierte die Ausweise.
The guard at the entrance checked the IDs.

guard / station
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guard
A person or group of people (e.g., soldiers, police officers) whose job is to guard and protect a place or someone.
Die Wache am Eingang kontrollierte die Ausweise.
The guard at the entrance checked the IDs.

'Die Wache' (the guard / station) is closely related to the verb 'wachen' (to stay awake, to watch) and the adjective 'wach' (awake). All of these words relate to the idea of alertness and protection.
Der Hund wacht die ganze Nacht vor der Tür.
The dog keeps watch in front of the door all night.
You can recognize the meaning of 'Wache' from context: if it refers to a person, they often stand at a place ('die Wache am Tor' — the guard at the gate). If it refers to the activity, the verb 'halten' or 'haben' is usually used ('Wache halten' — to stand guard). If it means the building, location prepositions are used ('auf der Wache' — at the station).
Die Wache kontrollierte jeden, der das Gebäude betrat.
The guard checked everyone who entered the building.
To express the activity of guarding, the fixed expression 'Wache halten' (to stand guard) or 'Wache haben' (to be on guard duty) is almost always used. The noun appears here without an article.
Wer hält heute Nacht Wache?
Who is standing guard tonight?
When 'die Wache' means a building (e.g., a police station), pay attention to the prepositions: 'auf der Wache sein' (to be at the station) describes the location, while 'zur Wache gehen' (to go to the station) indicates direction.
Er musste seinen verlorenen Geldbeutel auf der Wache abholen.
He had to pick up his lost wallet at the station.
service / duty
'der Dienst' and 'die Wache' can both refer to a working period or duty. 'Dienst' is more general: you can be on duty in a hospital, with the police, in the military, or in many other professions. 'Wache' is more specific and has to do with guarding and protecting. It can refer to the group of people keeping watch, or to the period of time when someone is on guard duty. The words are easy to confuse because a 'Wache' can also be a type of 'Dienst'. When you are talking generally about work or a shift, 'Dienst' fits. When someone is guarding a place, 'Wache' is more precise.

Vor dem Eingang stand die Wache die ganze Nacht.
The guard stood in front of the entrance all night.
Am Wochenende hat Frau Becker Dienst im Krankenhaus.
Ms. Becker is on duty at the hospital this weekend.