Why did Tom Riddle change his name to Voldemort? What was the point of Lord Voldemort, specifically, not Lord Tom or, Lord Riddle? Tom Riddle was his officially documented name, after all...
Voldemort despised his Muggle father, who was also named Tom Riddle for 1) being a Muggle, something Tom Riddle Sr. couldn't help, and 2) for leaving his mother Merope Gaunt while she was still pregnant with Tom Riddle. I interpret canon to mean Tom Riddle took on the moniker "Voldemort" mainly to distance himself from the name Tom Riddle and his Muggle side of the family. He also wanted a name that wizards and witches the world around would fear to hear or speak. "I am Lord Voldemort" from Chamber of Secrets is an anagram for Tom Marvolo Riddle.
‘You think I was going to use my filthy Muggle father’s name for ever? I, in whose veins runs the blood of Salazar Slytherin himself, through my mother’s side? I, keep the name of a foul, common Muggle, who abandoned me even before I was born, just because he found out his wife was a witch? No, Harry. I fashioned myself a new name, a name I knew wizards everywhere would one day fear to speak, when I had become the greatest sorcerer in the world!’
Chamber of Secrets - page 231 - UK Hardcover - chapter 17, The Heir of Slytherin
I researched it and apparently "Voldemort" means "flight of death" in French (Vol de mort). I confirmed this with Gilles, who is French, and he said it can also mean "theft of death." Either are appropriate when one reviews Voldemort's primary objective to avoid death, to essentially steal his own death away from the inevitability of mortality by creating Horcruxes. This is just my personal observation -- J.K. Rowling's university degree is in French, and she taught French for at least two years. I'm not sure how she didn't know that Voldemort means flight of death. I'm certainly not suggesting she's lying; it's just interesting.
Tom Riddle was his muggle father's name. His father left her mother when she was pregnant. So he wanted to shed it and become the most powerful wizard of all time.
"But not until a year after they were married. Tom Riddle left [Merope, Voldemort's mother], while she was still pregnant." "What went wrong?" asked Harry. "Why did the love potion stop working?" "Again, this is guesswork," said Dumbledore, "but I believe that Merope, who was deeply in love with her husband, could not bear to continue enslaving him by magical means. I believe that she made the choice to stop giving him the potion. Perhaps, besotted as she was, she had convinced herself that he would by now have fallen in love with her in return. Perhaps she thought he would stay for the baby's sake. If so, she was wrong on both counts. He left her, never saw her again, and never troubled to discover what became of his son."
Harry Potter and the Half-Blood Prince, pg. 214
As to why "Voldemort" exactly, that's more difficult. It is an anagram of Tom Riddle.
According to the author, Voldemort's name is an invented word.
It was his Muggle name. He hated his Muggle father, and by association all Muggles, so he sought disassociate himself from them as much as possible.
Tom Riddle also changed his name to 'lord Voldemort' because he felt he did not want anything to show the contempt that tied him to other people , anything that made him ordinary. This can be shown in the conversation between Dumbledore and young Riddle :
Dumbledore: - ask for Tom the barman, easy enough for you to remember as you share the same name (riddle gave an irritable twitch , as though trying to displace and irksome fly.) You dislike the name 'Tom' ?'
Riddle: - There are alot of toms,' muttered riddle
Tom wished to be different, special and seperate from the others.
I believe Riddle Tom chose the name Voldemort because it was the perfect name. From the begining, if you follow Voldemort's timeline, you will find that he liked to entice fear in people. He liked working alone and, most importantly, he hated his father as far as hatred could go. Even though he wanted a different name, he wanted to craft it out with what he was given. That is one quality that he had: resourcefulness. He liked keeping souveneirs. Names were no exception. That's why he chose the name "Voldemort". It was full of power, it radiated immense strength, and it still had a link to his origins.