Free radicals are renegade substances. They are the great white sharks of the subatomic. These are not nice guys. Free radicals are molecules that are missing an electron, which makes them unstable. This makes them crazy. So what do they do? They rush around trying to steal an electron from some other molecule and in the process they bash into our fragile cell walls and tear apart normal atoms; they even damage the delicate genetic blueprint stored in our DNA. That's a lot of destruction. And we are getting hit with about ten thousand of these every single day.
Not everything these free radicals do is so very terrible. Some of it is actually pretty good. Without free radicals we wouldn't have plastic, bread wouldn't turn into toast, we wouldn't have any hydrogen peroxide (and so would have only natural blondes), we wouldn't have medicines that fight fungus like athlete's foot or that kill bacteria.
But the dark side of free radicals is what causes masterpieces to deteriorate, rubber to get brittle, cars to rust, apples to turn brown, butter to turn rancid, and us to age, sicken and die.