Names in the Temporal Lobe
This is a rough picture of how memories for particular names are distributed in the left temporal lobe (the right lobe has a different set of functions. This assumes we're looking at your basic right-handed person. In lefties things get more complicated).
How do we know this? A Harvard group has developed a spectacular PET scan approach, using scans they have performed on people who have had strokes. They have a technique that allows them to sum up the results of scans from several people with similar strokes, and plot just where their brain tissue was damaged. From many such scans they have been able to show that when people are unable to remember peoples' names, their stroke was usually at the temporal pole as shown here. Similarly, people who cannot remember the names of animals usually had strokes in the middle of the lobe, and those who cannot remember the names of tools, at the back of the lobe -- as shown.
Why might the brain be organized like this? Think about what might have been important to humans when they were just evolving away from chimpanzees : other humans in the social group (people's names); things you can kill to eat, and things that kill you (animal names); and things you can use to get food or carry water or make shelter (tool names). This part of the brain enlarged rapidly at that time in human evolution. For a superb account of this period in time, from our human perspective, read Dr. Diamond's The Third Chimpanzee.