When you're using Google Spreadsheets, you're relying on Google to store your work on their servers. This may raise some concerns about privacy and security; all your data is in Google's hands.
These sound like reasonable concerns, but Google says you shouldn't worry. Although Google Spreadsheets stores your spreadsheets on their servers, they do not collect other personal information about you. In addition, Google uses a secure authentication method to control access to any spreadsheet you create. While you can grant others access to share your spreadsheets, those spreadsheets are private by default. Unless you share a spreadsheet URL, no one else can view that spreadsheet.