RG-6/U is a common type of  coaxial cable used in a wide variety of residential and commercial applications. The term "RG-6" itself is quite generic and refers to a wide variety of cable designs, which differ from one another in shielding characteristics, center conductor composition, and dielectric type. "RG" was originally a unit indicator for bulk radio frequency (RF) cable in the U.S. military's Joint Electronics Type Designation System. The suffix "/U" means “for general utility use.” The number was assigned sequentially. The "RG" unit indicator is no longer part of the JETDS system (MIL-STD-196E) and cable sold today under the RG-6 label does not necessarily meet military specifications. in practice, the term "RG-6" is generally used to refer to coaxial cables with an 18 AWG center conductor and 75 ohmcharacteristic impedance.

The most commonly-recognized variety of RG-6 is cable television(CATV) distribution coax, used to route cable television signals to and within homes, and RG-6 type cables have become the standard for CATV, mostly replacing the smaller RG-59, in recent years. CATV distribution coax typically has a copper-coated steel center conductor and a combination aluminum foil/aluminum braid shield, typically with low coverage (about 60%). RG-6 type cables are also used in professional video applications, carrying either baseband analog video signals or serial digital interface (SDI) signals; in these applications, the center conductor is ordinarily solid copper, the shielding is much heavier (typically aluminum foil/95% copper braid), and tolerances are more tightly controlled, to improve impedance stability.

RG-6 cables typically are fitted with various types of connector at each end; in CATV distribution applications, these are typically F connector style; in professional baseband video, BNC connectors; and in consumer a/v applications other than RF and CATV, RCA plugs.

Source from http://en.wikipedia.org/wiki/RG/6.