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 usually ends with -um (Latin) or -ον -on (Greek), matching the masculine accusative.
The Wiktionary appendix Second declension contains more detailed information and full paradigm tables for the Latin second declension.