Original: They are similar to the declension system in Latin: the first declension are all words on -a, the vast majority of which are feminine; the second declension are mostly masculine and neuter words ending with a consonant.
Simplified: They are very much like the declensions in Latin: the first declension are all words on -a, almost all of them are feminine; the second declension are mostly masculine and neuter words ending with a consonant.