A drip edge is a piece of metal, plastic or vinyl that is nailed at the bottom of the roof edge, or eaves, to facilitate proper water drainage. It allows for the water to drain into the gutters or cleanly off of the roof rather than running down the roof edge and down the fascia, which will eventually lead to rot. The drip edge is one of the first things attached to the roof and should be installed underneath the roofing felt with about inch of overhang extending past the fascia and hanging just over the gutters. A drip edge can range in materials and thickness, but typically a roof drip edge is most commonly made from aluminum.
In the second picture to the right a well meaning cable installer ran cable right alongside the drip edge. This allowed water to circumvent the drip edge by running down the cable to the fascia. Over time water warped and began to rot the wood requiring the home owner to make repairs.