Sex addiction has transformed over the years, but generally it is defined as an addiction to sexual behavior that can be expressed through multiple partners, anonymous sex, prostitution, porn (including Internet porn), and dangerous/deviant sexual behavior.
Most recently sex addiction has expressed itself through the rapid growth of Internet Porn. Internet porn can be accessed by videos, picture, and even live web cam sex.
Sex addicts exhibit the same behavior as addicts of substances. Essentially the sex addict is no different than the coke or alcohol addict and forms a tolerance and dependence just as well.
Sex addiction has become more and more rampant the more society condones and even glorifies sex. The more this is done, the more sex addiction will appear.