Airplanes are like cars, each one has a specific and unique license plate number, but on aircraft the number is painted on the aircraft, usually near the tail. (Hence, the term "tail number.") Each country has a registration prefix set aside for their exclusive use. Aircraft registered in the United Kingdom carry the "G-" prefix, France has the "F-" prefix, and closer to home, Canadian airplanes carry a "C-" prefix. Some countries, like the ones I mentioned, use all letters in their registrations, but the U.S. uses the "N-" prefix followed by a combination of numbers and letters. I have no earthy idea why we didn't get the "U" designation, but I think it is because the Soviet Union originally used this before World War II. If anyone knows for sure, please post the answer. By the way, the Federal Aviation Administration's (FAA) guidelines for registration numbers can be found here.
The other airplane freaks (besides me) among us know that we normally use "WN" (for our IATA airline code), "SW" or "SA" as our standard registration suffixes for our aircraft. When we went to the FAA to reserve the "N700xx" block of registrations for our Boeing 737-700s, we found that a lot of other aircraft owners like to use 700 numbers in their own registrations. No doubt, high rollers especially like N711xx. As a result, we had to scramble to find suffixes that had not already been taken because some folks had beat us to the punch and were already using our preferred suffixes. So, we turned to the initials of some of our VIPs, and in addition to Herb's (HK) and Colleen's (CB) initials, we used "GS" for Gene Stewart our 737-700 Project Director, "JW" for Jim Wimberly, our now retired Executive Vice President Aircraft Operations (and fellow aircraft nut), "RR" for Ron Ricks, Senior Vice President Law, Airports & Public Affairs, and "GB" for Gary Barron, our former Executive Vice President Operations. We also used "LV" as a contraction for LUV. As the 737-700 fleet grew, we ran out of the N700xx numbers, and next went to N400xx and are currently in the N200xx block. Incidentally, our retired 737-200s used numbers from 20 to about 105, our 737-300s are in the 300 and 600 blocks, and our 737-500s use the 500 block. Unlike some other airlines that assign additional "fleet numbers" (which don't always correspond to the registration) to their aircraft for internal identification, we use the N-number for internal identification.