From the GPF FAQ:
What's a "Faultie?" I keep seeing that term used around this site....
A Faultie is a wonderful, brilliant, intelligent, and often highly creative individual who just happens to be a devoted fan of GPF. There are no other requirements for the title, which means Faulties can come from incredibly diverse and interesting backgrounds. This diversity gives Faulties their strength, while the common bond of reading and enjoying GPF draws ordinarily uncommon elements into one cohesive body of readers.
There is no initiation to become a Faultie; simply read the strip and enjoy it. Some denizens on the forum may experience a brief hazing ritual called "poking the newbie," but this is largely harmless. One word of warning, however: Simply reading the strip may become habit forming, and some individuals have exhibited extreme cases of obsession, including caring so deeply for the characters that they flame the cartoonist when something bad happens to them, or posting excessively to our forum. There is no cure for these symptoms... the main comfort of those afflicted with this condition is to communicate with other Faulties and share their joys and sorrows.