Sequencing refers to programming a computer to execute a song in a live situation, different from recording which is just playback. The computer "plays" the instrument desired via Musical Instrument Digital Interface or "midi" for short. It's like having a ghost drummer who plays what you want him to. Think of it like those old player pianos from the 1800's. It's used if you want drum parts played in a song and you don't have the drummer to do it. The drawback is that sequencers cannot improvise or correct mistakes, once they're unleashed, they play their parts exactly as programmed. Even industrial megastars like Trent reznar from Nine inch nails don't rely too much on them for that fact, he has an army (about 9 guys) in the background playing synthsizers instead of sequencing his stuff. It is a very powerful tool though if you know what you're doing.