Much like English, words within a Latin sentence have different purposes in order to form a complete sentence. The 'cases' in Latin are a way to describe and identify the various functions of nouns only. In total, there are six cases.

THE NOMINATIVE: This refers to the subject of the sentence, or the noun that is completing the action of the verb. 

Eg. cives ad portam processerunt.

       The citizens proceeded to the gate.

Here, "cives" (the citizens) is the nominative noun because they are the ones proceeding to the gate.

THE ACCUSATIVE: This refers to the direct object of the sentence, or the noun that has the action done to it.

Eg. cives ad portam processerunt.

       The citizens proceeded to the gate.

Here, "portam" (the gate) is the accusative noun becuase it is being proceeded to by the citizens.

THE GENITIVE: Genitive nouns are used in Latin to indicate possession, or to show who the noun belongs to. It is useful to remember that the genitive should be translated using the english word 'of'.

Eg. filia senatoris

      The daughter of the senator/the senator's daughter

Here, "senatoris" (the senator) is the noun in the genitive case, demonstrating that the daughter belongs to him.

THE DATIVE: The dative is the indirect object of the sentence. This means that the action of the sentence is completed for the dative noun. Dative nouns are often translated using the English words "to" or "for".

Eg. peur canem amico dedit.

      The boy gave a dog to his friend.

Here, the "amico" (friend) is the dative noun, because the boy is completing the action for the benefit of his friend.

THE ABLATIVE CASE: Nouns in the ablative are often translated using the English words "by", "with" or "from", and so often explain how an object is used to achieve the completion of the verb.

Eg. miles hostem gladio necavit.

      The soldier killed the enemy with his sword.

Here, "gladio" (sword) is the noun that is in the ablative, since it is the noun with which the soldier kills his enemy.

THE VOCATIVE: The use of the vocative is perhaps the most straighforward, since it is used to address another noun directly, often in direct speech.

Eg. "domine, Clemens est in atrio," Grumio dixit.

      "Master, Clemens is in the atrium," said Grumio.

Here, "domine" (master) is the noun in the vocative because it is the master who is being directly addressed and named by the speaker, Grumio.