Digging into the documentation you'll find a more detailed description of the API:
It's important to realize that the only data source that can currently be read using the Visualization API (a "compliant data source") is a Google Spreadsheet and the API doesn't yet define how to implement a compliant data source. Since Google Spreadsheets already has a RESTful GData-based API (Google Spreadsheets API Profile), you might wonder what's the point about having yet another API to access Google Spreadsheet. The point is: the Visualization API is meant for tabular data from any source, not just from Google Spreadsheets. In the specific case of visualizing data, you won't need to know the details of the Google Spreadsheets API to get at data from a Google Spreadsheet.
The Visualization API can be used to display data from data sources in a variety of contexts, most prominently in Google Visualization Gadgets that can be displayed in iGoogle or a Google Spreadsheet (akin to embedding a chart inside an Excel spreadsheet). A quick way to see the Visualization API in action is Google Visualization API Gadget Gallery, a showcase for what people have done with these gadgets so far. Follow the instructions instructions on using Visualization Gadgets to apply these gadgets to your own Google Spreadsheet. To learn how to program with the Visualization API, start by working through "Hello World" example of the API and the tutorial on Gadget Extensions - Google Visualization API - Google Code.