Information security is a top priority at Google, and dedicated teams are employed with experts in their fields to handle these important areas. Your files are, by default, set to private, but you can choose to publish them to the Web or invite collaborators or viewers. Your files will not appear in Google Web Search results, but published files may appear in other search engines.
If you are working in a Google Apps domain, your files are, by default, set to your domain's default setting. This setting might not be private, but you can make your docs private when you create or upload a doc or file.
Additionally, to help keep your data private, it's important to have a strong password, to not share that password with others, and to understand the sharing options in Google Docs.