of the most common theme in Fantasy literature is names and the Power
they command. Sometimes they are magical True Names – such as in The Inheritance Cycle by Christopher Paolini, Ursula K. Le Guin's Earthsea Cycle, The Bartimaeus Sequence by Jonathan Stroud, and the Books of Pellinor by Alison Croggon – to name a few, where to know a person or things True Name gives you great power over it.
"Who knows a man's name, holds that man's life in his keeping." - Ursula K. Le Guin
"To change this rock into a jewel, you must change its true name." - Ursula K. Le Guin
They are effectively the words in which spells are cast, the Truth behind the names being what gives the spells power, which means there is almost-invariably a language in which they are a part. In the Earthsea and Inheritance Cycles is it the Language of Making and the Ancient Language respectively, in the Pellinor Quartet tis the Speech, but no language at all in Bartimaeus (names simply have power here and that's the end of it). This theme occurs often enough that any inveterate Fantasy reader is familiar with it, for it reaches far beyond the four named series' above.
Then there are names in their other sense, bearing a different yet still potent power. Recall in Harry Potter how much fear in invested in Voldemort's name, how good wizards refereed to him as You-Know-Who and He-Who-Must-Not-Be-Named, while the Death Eaters and their ilk called him simply the Dark Lord because their master forbade them call him Voldemort.
"Call him Voldemort, Harry. Always use the proper name for things. Fear of a name increases fear of the thing itself." - Albus Dumbledore
Well, Voldemort did this on purpose for he knew the power names have and, ironically perhaps, hated his birth name to the extent that it gave Harry and Dumbledore an emotional edge over him. Perhaps the name Tom Riddle angered him because it reminded him of his past, his old fears and vulnerabilities, so he crafted a new name for his new self and them cloaked it in fear so it could never be used against him. Who knows? But those such as Voldemort who rule by fear and death as a rule fear death and the erosion of that fear more than anything in the world. Indeed, fear of death was Voldemort's undying fixation. More to the point, it was no accident that Voldemort's past as Tom Riddle proved so crucial in Harry and Dumbledore's efforts to defeat him for, as Terry Pratchett once said, "Before you can kill the monster you have to say its name." Indeed, characters with many and secret names is even more common than the True Name trope, and Voldemort is far from the first Evil to fall because the Heroes of Light dug deeply into their past. Perhaps that was another reason he took the name Voldemort, to cover his tracks.However, the importance of recognizing something's name and nature goes beyond fighting the Evil thing. "The wise man knows his own name," as said Patricia A. McKillip, and just as if not more often the main protagonists have to come to terms with a name that goes hand-in-glove with a destiny they never asked for or wanted. Morgon of Hed from McKillip's Riddle-Master Trilogy where the quotes above and below come from is a perfect example of this, but almost all Chosen Ones feel the same. Other beyond excellent examples are ta'verens Rand al'Thor, Mat Cauthon and Perrin Aybara from Robert Jordan's The Wheel of Time, and Maerad of Pellinor from Alison Croggon's Books of Pellinor.
"If you have no faith in yourself, then have faith in the things you call truth. You know what must be done. You may not have courage or trust or understanding or the will to do it, but you know what must be done. You can't turn back. There is now answer behind you. You fear what you cannot name. So look at it and find a name for it. Turn your face forward and learn. Do what must be done." - Patricia A. McKillip
These are of course FAR from the only poignant name-related quotes, so if you need more then I highly recommend ye look up Terry Pratchett since he created a small host of them. In the meantime, you can read the expanded version of this post on the page of the same name.