"Case" refers to the endings of nouns that indicate their function in a sentence. The Nominative Case indicatese a subject (the noun doing the action); the Accusative Case indicates a direct object (the noun that has the action done to it).

"Declension" refers to the groups of nouns that have similar endings in the different cases.

Declension	nom. / acc.
1st Decl.:	-a / -am
2nd Decl.:	-us / -um
3rd Decl.:	-- / -em