The power of SharePoint Enterprise Search is that it does not only query data stored in one location but can pull data from a variety of sources, a technique known as "federated searching".
When a user performs a search, different search engines are queried using the "Open Search" standard. This allows various data sources including SharePoint sites, desktops, line-of-business applications (e.g. Lotus Notes), and even users' brains to be searched and the results displayed in one aggregated results list.
Note
Don't worry about SharePoint searching our minds. This is not as scary as it sounds and requires no brain implants. SharePoint's people search merely identifies which brain the information is stored in and allows the data to be retrieved in a traditional manner, such as a telephone conversation or email exchange.