What makes a ragtime song a ragtime song? Which are the key and defining elements of ragtime? What differentiates it from other music from the era?
The defining characteristic of ragtime music is a specific type of syncopation in which melodic accents occur between metrical beats. This results in a melody that seems to be avoiding some metrical beats of the accompaniment by emphasizing notes that either anticipate or follow the beat ("a rhythmic base of metric affirmation, and a melody of metric denial" -Wikipedia
Basically, it's just a specific form of syncopation where accents do not fall on a beat.