In the world of
body modification, this is the process of stretching out a piercing in order to wear larger
jewelry. While some people simply stretch their
fistula by roughly shoving in the new jewelry, the preferred method is to use a
taper, or small, sterile rod which gradually increases in girth to stretch the hole. Generally you would use a
lubricant in order to ease the process (I've found that Vagisil works pretty well). Some people prefer to ease the taper through slowly and carefully, but I haven't found many professional piercers who endorse this practice. Usually, you are requested to breath in, then breath out, then breath in once more, and the taper is shoved through quickly and with relatively minor
pain. However, if you gauge out too quickly you can damage your piercing, and possibly suffer
blowout or even a
torn lobe. Generally, waiting several months to stretch a piercing is best, and shouldn't be a very big hassle, unless you are completely
obsessed with the concept.
Jewelry sizes are measured (in America anyway) by gauges. A higher gauge represents a smaller piece of jewelry. The typical spectrum of sizes range from 20 gauge (very dainty) to 00 gauge (quite fat). Many people stretch their piercings beyond 00, but after this point they generally measure in normal increments of measurement(after 00, the next step up is generally 7/16ths of an inch). The person with the largest gauge ears (that i've ever heard of/seen) is Bear, a piercer who works at True Blue tattoo studio in Austin, Texas. His ears are currently at 6 inches, which translates into lobes that touch his shoulders.
Gauging is a process that was absorbed into American and other cultures from Africa, where it has been practiced for generations. A good place on the web to learn more and view a lot of pictures, including some of the aforementioned Bear (who is an awfully nice guy), would be www.bme.freeq.com, a website devoted to body modification in all its facets.