Profanity, also known as bad language, swearing, cursing, or cussing, is the usage of words which are considered to be offensive or abusive. Its use is taboo in polite or formal environments, and more commonly accepted in informal contexts. The word "profanity" comes from the word "profane", which means that which is not sacred.
Profane words tend to involve certain near-universal subjects: filth, particularly human waste; forbidden sexual practices; and blasphemous treatment of the sacred. However, it is the words themselves, and not the concepts they represent, which cause offense, as there are numerous words to signify any taboo subject, with varying levels of acceptability. Among profane words, there exist at least two levels of offensiveness. There are also, in many languages including English, euphemisms such as darn, drat, gosh, frap and so on.
Over time, the perceived offensiveness of some expressions has changed. Up to the twentieth century the most offensive terms were religious. Many exclamations we find completely innoffensive are euphemistic renditions of blasphemy. The twentieth century saw a shift to sexual terms as the most offensive. Recently racial terms have grown in their offensiveness (the "N" word was commonplace and acceptable until the mid-nineteenth century, it is now too offensive even for use in male-only settings) due to political correctness. 
- Why Do We Love Profanity?, Brent Bozell, GOPUSA, January 30, 2009