Many web hosting services offer so-called unlimited or unmetered service for whatever amount of bandwidth, disk storage and sites you use. It's important to understand that most terms of service actually do limit the definition of "unlimited" to what's considered reasonable use. The bottom line is simple: if you're building a pretty basic website, unlimited means you don't need to worry. But if you're trying to do something excessive (or illegal, immoral or fattening), the fine print in the terms of service will trigger, and you'll either be asked to spend more or go elsewhere.
In housing terms, VPS hosting is like renting your own apartment in a larger building. You're much more isolated than in the roommate situation mentioned above; it's still possible that a neighboring apartment could causes annoyance for you, but far less likely. In web hosting terms, Site A's traffic surge won't have nearly as much impact on Site B or Site C. As you'd expect, VPS hosting costs more than shared hosting. You'll pay roughly $20 to $60 per month.