Pads are designed to fill out a track. In times past they were usually organ or organ-like synths (hammond being a big one iirc), but in more modern times they tend to be entirely synthesized sounds. Think of them as doing the bass's job, but a higher harmony to the bass (in job, rarely in actual notes). Instead of holding the low end of the track and percussion together, they help to blend the sounds in the higher registers. Generally they're very spacey, ambient noises and almost always in chords with a lot of audible fuzziness (like reverb).