A people mover is traditionally defined as a light rail system, usually elevated, almost always automated. (In some areas, it's become a generic term for a lot of different types of commuter rail line.) They were designed to accommodate places where a more complicated rail line might be overkill. Basically, it's a monorail, a friendly alternative to faster, more complex, more utilitarian subways and elevated rail lines. It seems cleaner and, frankly, happier, than descending a staircase to a dark, fetid underground subway platform.
Generally, people mover systems work best over limited areas, like the small, driverless airport trams that shuttle passengers between terminals. They also work well if they're meant to convey a sense of nostalgia, like at Walt Disney World. They aren't always particularly competitive when they're meant to supplement, or compete with, other forms of urban mass transit, like subways or buses. Just ask the city of Detroit, where the Detroit People Mover serves an under-populated downtown area, circling around 3 miles (4.8 kilometers) of tourist attractions and commercial districts, with costs far outweighing collected fares. In this struggling city, taxpayers waste a few bucks for each mile ridden by a passenger. And studies show that most passengers are tourists in the city, so the people mover isn't even moving the people who actually live there.
Not trying to bash Detroit, here -- after all, any poorly planned transit system is likely to fail, and even the well-designed and popular ones aren't often revenue generators. Miami's Metromover People Mover, which connects busy parts of the city, is a good example of how the system can succeed. The people mover concept just goes to show how a mode of transportation might seem somewhat promising and look really cool, but that doesn't guarantee success.