What is the second declension?

Nouns of the second declension are normally masculine or neuter. 

Masculine:

Singular:

nominative: -us 

genitive: -i 

dative: -o

accusative: -um

vocative: -e or -i 

ablative: -o

Plural: 

nominative: -i

genitive: -orum 

dative: -is

accusative: -os 

vocative: -i 

ablative: -is