The German language has four different cases.
Every case has a certain task to fulfill within a sentence.

First case: nominative (abbr. NOM)

This case marks the subject of a German sentence.
You will encounter German nouns in the nominative in every dictionary.
When you are looking for a noun in the nominative in a sentence, you use the question: Wer oder was? (‘who or what?’)
 
Example:
Der deutsche Mann trinkt gerne Bier.
The German man likes to drink beer.
→ Wer trinkt gerne Bier? Der deutsche Mann.
→ Who likes to drink beer? The German man.

Second case: genitive (abbr: GEN)

The genitive marks possession.
If you are looking for a genitive in a German sentence, you ask: Wessen? (‘whose?’)
 
Example:
Christines Papa ist nett.
Christine’s dad is nice.
→ Wessen Papa ist nett? Christines.
→ Whose dad is nice? Christine’s.

Third case: dative (abbr. DAT)

This case marks the indirect object in a sentence.
The question of your choice for that case is: Wem? (‘whom?’)
 
Example:
Ich gebe meiner Nichte ein Geschenk.
I give a present to my niece.
→ Wem gebe ich ein Geschenk? Meiner Nichte.
→ Whom do I give a present? My niece.

Fourth case: Accusative (abbr. ACC)

The accusative marks the object in a sentence.
If you are searching for a noun in the accusative in a German sentence, you ask: Wen oder was? (‘who or what?’)
 
Example:
Marc liebt die deutsche Sprache.
Marc loves the German language.
→ Wen oder was liebt Marc? Die deutsche Sprache.
→ What does Marc love? The German language.

Maybe you already noticed?
The cases are marked by different ending in the nouns.
German nouns follow a certain system and rules in taking the endings for the different cases.
Linguists like to call that system ‘declension’.
When learning German, knowing these rules is key.
So, please, take a close look at the following chapter.