
der Lenker
handlebar
The bar with grips used to steer a bicycle, moped, or motorcycle.
Halte den Lenker mit beiden Händen fest, wenn du bergab fährst.
Hold the handlebar firmly with both hands when you ride downhill.
handlebar / driver / leader
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handlebar
The bar with grips used to steer a bicycle, moped, or motorcycle.
Halte den Lenker mit beiden Händen fest, wenn du bergab fährst.
Hold the handlebar firmly with both hands when you ride downhill.

driver
A formal term for the person who steers a vehicle; the driver.
Der Lenker des Wagens war für den Unfall verantwortlich.
The driver of the car was responsible for the accident.

leader / guiding force
A person who decisively directs and influences the fate or development of something or someone.
Er gilt als der eigentliche Lenker hinter der neuen politischen Bewegung.
He is considered the real guiding force behind the new political movement.
The word 'Lenker' (handlebar / driver / leader) is derived from the verb 'lenken' (to steer, to guide). The ending '-er' can refer both to the tool for an action (the steering bar) and to the person who acts (the driver, the leader).
Er lenkte das Gespräch geschickt auf ein anderes Thema.
He skillfully steered the conversation to another topic.
The meaning becomes clear from the context. With a 'Fahrrad' (bicycle), it means the handlebar. With an 'Unfall' (accident), it means the driver. With 'Geschicke' (fate / affairs) or 'Politik' (politics), it means a leading figure.
Der Lenker des Fahrrads verlor die Kontrolle.
The rider of the bicycle lost control of the handlebar.
The term 'Lenker' (driver) for a driver is very formal. It is mainly used in official language, for example by the police or in news reports.
Die Polizei befragt den Lenker des Lastwagens.
The police are questioning the driver of the truck.
In a figurative, elevated meaning, 'Lenker' (leader / guiding force) describes a person who has great influence and steers the development of something, for example the 'Geschicke eines Landes' (the fate of a country).
Bismarck gilt als Lenker der deutschen Politik im 19. Jahrhundert.
Bismarck is considered a guiding force in German politics in the 19th century.
driver
Fahrer and Lenker can both refer to the person at the wheel of a vehicle. Fahrer is the usual term in everyday language, while Lenker sounds more formal and is used especially in official or factual contexts. Usually use Fahrer, unless a formal term is appropriate.

Der Lenker muss seinen Führerschein bei der Kontrolle vorzeigen.
The driver has to present his driver's license during the inspection.
Der Fahrer muss seinen Führerschein bei der Kontrolle zeigen.
The driver has to show his driver's license during the inspection.