Leg warmers are a sock-like garment that are worn underneath cycling shorts. What happens some days (particularly on longer riders) is that you start out very cold, but as both you and the day around you warm up, you no longer require all the layers you did when you set out.
Leg warmers are great for keeping your legs warm as well as keeping your skin out of the sun. If you heat up during a ride you can easily push them up or take them off, and they pack nice and small to stow away.
Enter warmers, which are most commonly worn on the arms and knees, and simply slide on and off the exposed skin when required. This allows you to stay warm early on, then as the sun comes out you simply pull off said warmers on the go, without needing to stop the bike. They're then small enough to easily tuck away into your jersey pockets for the rest of the ride.