This doesn't, as the title would suggest, refer to the title by which pens wish to be called, but rather to the practice of pretending you're someone else when writing something to be published. This is also referred to as using a pseudonym or a nom de plume.
Many of the most famous books of all time were not actually written by their supposed authors. 'Mark Twain' aka Samuel Clemens isn't really an author but a measurement of depth of the Mississippi River, USA, used on the riverboats in the 19th Century.
Novel writing isn't the only area in which people change their names. Actors often make up better, more interesting names for themselves, as do those in the world of music. This is really just a ploy to sell more records or get more film parts. Famous actors/personalities who have changed their names include:
Joan Crawford née Lucille Leseuer
Cary Grant né Archibald Leach
Richard Burton né Richard Jenkins
Judy Garland née Frances Gumm
Elton John né Reginald Dwight
Whys and Wherefores
The reasons for changing your name for publishing your work are varied, but some of the more popular ones are listed below:
You don't like the sound of your own name.
You can't get your book published because you're too young, famous in another field or, in times past, female.
You have written about something illegal that you have done and are on the run from the police.
You've insulted a major religion in your work and don't want to face the consequences.
You want to remain anonymous to avoid getting fan-mail, stalkers or the sack.
Your name sounds stupid and no-one would take you seriously as a result.
Your real name is unpronounceable by 99% of the world's population.
Pen names are becoming increasingly widespread as the media continues to grow in size, if this trend continues no-one will be sure who they are really talking to.