der Unterschied
difference
A feature or characteristic that makes two or more people, things, or facts different from one another.
Was ist der Unterschied zwischen einem Laptop und einem Tablet?
What is the difference between a laptop and a tablet?

difference
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difference
A feature or characteristic that makes two or more people, things, or facts different from one another.
Was ist der Unterschied zwischen einem Laptop und einem Tablet?
What is the difference between a laptop and a tablet?

The noun 'der Unterschied' (difference) is derived directly from the verb 'unterscheiden' (to distinguish). When you distinguish between two things, you find the difference between them.
Können Sie den Unterschied zwischen diesen beiden Bildern erkennen?
Can you spot the difference between these two pictures?
To compare two things, the preposition 'zwischen' (between) is almost always used, followed by two nouns in the dative. The pattern is: 'der Unterschied zwischen [thing 1 in the dative] und [thing 2 in the dative]' (the difference between [thing 1] and [thing 2]).
Es gibt einen großen Unterschied zwischen dem Leben in der Stadt und auf dem Land.
There is a big difference between life in the city and in the countryside.
The idiom 'einen Unterschied machen' (to make a difference) means to have a noticeable effect. It is often used to emphasize the importance of an action or contribution.
Dein Engagement in diesem Projekt hat wirklich einen Unterschied gemacht.
Your commitment to this project really made a difference.
The expression 'im Unterschied zu' (unlike / in contrast to) introduces a contrast and is always used with the dative. If the phrase appears at the beginning of the sentence, the verb follows directly after it (inversion).
Im Unterschied zu gestern scheint heute die Sonne.
Unlike yesterday, the sun is shining today.
Common adjectives that describe 'Unterschied' (difference) are 'groß' (big), 'klein' (small), 'kaum ein' (hardly any), 'wesentlich' (essential), or 'entscheidend' (decisive).
Der wesentliche Unterschied liegt im Detail.
The essential difference lies in the detail.
influence
'der Unterschied' and 'der Einfluss' can both be important when comparing two situations or outcomes. 'der Unterschied' says that two things are not the same. So it is about a comparison. 'der Einfluss' says that a person, thing, or situation changes something or has an effect. The words can be especially confusing in sentences like 'Das macht einen Unterschied' (That makes a difference) and 'Das hat Einfluss' (That has an influence). In those cases, both mean that something is important for the result. The difference is: an 'Unterschied' shows what is different; an 'Einfluss' explains what causes something to become different.

Zwischen den beiden Kursen gibt es einen großen Unterschied.
There is a big difference between the two courses.
Vor der Prüfung hatte der Stress einen starken Einfluss auf ihr Lernen.
Before the exam, stress had a strong influence on her studying.