The "pop" of Pop music is, more than likely, an abbreviation for popular music. Unlike much rock music, whose lyrics tend to be very anti-establishment, even outright nihilistic, pop music generally focuses on having fun. Themes are generally about dancing and partying and essentially living the good life, but occasionally, the subject matter can be more serious. Generally, pop songs are designed so that the tune will get stuck in the listener's head.
Like all musical genres, it is very hard to nail down a perfect example. Pop is even more difficult, since it has a lot of cross-over status. Virtually any artist of any genre can produce a "pop hit" by creating a big impression on the charts. For example, Michael Jackson has been called "The King of Pop", yet most will argue that his style was more R&B or Soul. The "Queen of Pop" Madonna would probably fall more under this category, as would the "Pop Princess" Britney Spears. The recent success of Justin Timberlake has made him the "Prince of Pop".
Many have criticized that pop music is more of an invention of record companies to sell more albums, than it is an artistic movement. The fact that record companies package pop artists for maximum promotion and sales is often a sign for rock fans that true talent is being ignored somewhere. There was much debate in the late nineties over 'boy bands' like N'Sync, The Backstreet Boys, and 98 Degrees, who, despite phenomenal record sales, were labeled 'no-talent' by the rest of the rock community. However, there are some pop artists, like Elvis Presley and the Beatles, whose works have passed the test of time. One thing is for sure, as long as there are record companies and Top Ten charts, there will always be Pop music.