Like punk bands, ska punk bands feature a drumset, bass, guitar and keyboards. There is a wide spectrum as far as bands that are more ska and those that are more punk, but those with a definite ska flavor usually include a horn line (trumpet, trombone and/or saxophone), vocal harmonies, auxiliary percussion and/or organ.
The guitar is typically heavily distorted in ska punk music, an element derived from the punk music side. The horn line is usually punchy and syncopated, with all of the horns accentuating the melodic line in harmonic unison. Reggae, a genre considered to be the inspiration for ska, has an influence on the bass line. Bassists in a ska punk band usually play a walking bass line and focus on arpeggios and scales. The overall sound of ska punk is clear and staccato, with easily understandable lyrics and clear synchronized lines shared between instruments.
Bands that lean more towards the punk side of ska punk music employ very fast tempos in their songs, as well as rapid eighth and sixteenth notes in the drumset parts, with all four beats emphasized. Songs that are more ska than reggae still utilize a fast tempo--faster than reggae--but with a slightly more relaxed feel that emphasizes beats two and four, like reggae.