I think it's the rubber housing ("boot") that goes around the front-wheel assembly that allows the front wheels to rotate at different speeds while under power and turning. CV boots are "consumables" in that they eventually dry out, crack, and need to be replaced, because they hold in the grease that keeps the CV (constant velocity?) assembly from going kaput. It's a legit maintenance item.
I had an Integra that had a CV boot puke out. When it was replaced, they ended up finding that it was cheaper (parts+labor) to replace the whole axle because the boot was fused to the CV joint, or something.
