Direct burial cable is a special type cable that is designed to be run in a trench underground without the use of conduit to surround it. The electrical wires are encased in a thermoplastic sheath that seals out moisture and protects the wires within.
Direct burial cable, often called exterior-rated cable or UF cable (short for underground feeder), comes in a gray jacket and is approved, when installed at the proper depth, location, and properly sized, for burial in a trench. For instance, you may want to feed a small pole light near your flower garden.
The difference between standard non-metallic sheathed cable (NM) and underground feeder (UF) is very noticeable. Notice in the photo how the standard NM cable simply covers the group of wires and the UF cable actually coats each conductor individually. This encasement protects each and every wire from the other and does not allow moisture or other external elements to reap havoc on the wires.
As a general rule, UF cable should be buried at least 18 inches, but check local codes and regulations before beginning your project. As a rule of safety, always be sure the power is shut off to anything electrical before working with electrical equipment , devices, or wiring.