Second declension
The second declension is a category of nouns in Latin and Greek with similar case formation. In particular, these nouns are thematic, with an original o in most of their forms. In Classical Latin, the short o of the nominative and accusative singular became u.
Both Latin and Greek have two basic classes of second-declension nouns: masculine and neuter. Most masculines have -us (Latin) or -ος -os (Greek) in the nominative singular, except for the r-stem nouns in Latin, and the "Attic" declension and contracted declension in Attic Greek. The neuter nominative/accusative singular ends with -um (Latin) or -ον -on (Greek), matching the masculine accusative.
See also
For specifics on the second declension as it appears in Latin and Greek, see the appropriate sections in Latin declension and Ancient Greek nouns.
The Wiktionary appendix Second declension contains more detailed information and full paradigm tables for the Latin second declension.