der Dienst
duty / shift
The professional activity or work that someone performs at a particular time; working hours or a shift.
Ich habe heute von 8 bis 16 Uhr Dienst.
I'm on duty today from 8 a.m. to 4 p.m.

service / duty
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duty / shift
The professional activity or work that someone performs at a particular time; working hours or a shift.
Ich habe heute von 8 bis 16 Uhr Dienst.
I'm on duty today from 8 a.m. to 4 p.m.

'Dienst' (service/duty) is the noun related to the verb 'dienen' (to serve). All meanings of 'Dienst' have to do with the basic idea of serving someone or something, or providing a service.
Ein guter Kellner dient seinen Gästen mit Aufmerksamkeit.
A good waiter serves his guests attentively.
When referring to working hours, the collocations 'Dienst haben' (to be on duty), 'im Dienst sein' (to be on duty), and 'außer Dienst sein' (to be off duty) are very common. It is also often used in compounds such as 'Frühdienst' (early shift) or 'Nachtdienst' (night shift).
Die Ärztin hat heute Nachtdienst.
The doctor is on night duty today.
In the sense of a service, 'Dienst' is often found in business contexts. It is at the core of words such as 'Dienstleistung' (service), 'Kundendienst' (customer service), or 'Lieferservice' (delivery service).
Der Reparaturservice dieser Firma ist ausgezeichnet.
This company's repair service is excellent.
The meaning 'favor' occurs almost exclusively in the fixed expression 'jemandem einen Dienst erweisen' (to do someone a favor) or, more colloquially, 'jemandem einen Dienst tun' (to do someone a favor).
Du würdest mir einen großen Dienst erweisen, wenn du mich vom Bahnhof abholst.
You would be doing me a big favor if you picked me up from the station.
You can recognize this meaning from the context of state institutions such as the military ('Bundeswehr'), police, or fire department. The typical verb in this context is 'leisten' (to perform/do).
Er musste nach der Schule seinen Dienst bei der Armee leisten.
After school, he had to do his service in the army.
work
'die Arbeit' and 'der Dienst' can both refer to professional work. 'Arbeit' is the more general word for a job, employment, or what someone does professionally. 'Dienst' fits better when talking about a specific working time, a shift, or work in an organization, for example in a hospital, with the police, or in the military. The words can be confused because both have to do with paid work. When you are speaking generally about your job, 'Arbeit' is usually more natural. When you are talking about a shift or an assignment, 'Dienst' is more precise.

Lena hat morgen Dienst und kann deshalb nicht zum Treffen kommen.
Lena is on duty tomorrow, so she can't come to the meeting.
Nach der Arbeit fährt Mehmet direkt nach Hause.
After work, Mehmet drives straight home.