If you use the Web Network Version of the program, your database is stored in a secure location on a Software4Nonprofits.com server. All access to your data and other aspects of the server is encrypted, using the same types of secure protocols used for online banking etc.
Your data is protected by many levels of secret techniques and strong encryption, but ultimately the final level of security will always depend on securing the login email address and password you use to access the database through the program. (Anyone with that information could install the Web Network Version of the program and access your database. Virtually any web-based software has exactly this same problem!) That is why strong passwords are required for your account.
No security method is 100% unbreakable. Having said that, it would require a very determined and extremely highly skilled hacker to break through all of the levels of security built into the Web Network Version and be able to get access to your data or others' data, without knowing your login information.
Because no extremely valuable information like credit card numbers is stored in the ACCOUNTS database (or at least, you should not be doing so!) there is also probably very little incentive for high-level hackers to try to get at your data.
For those users who are in Canada and are concerned about implications of our privacy laws on whether you are allowed to transmit sensitive information out of the country, the Software4Nonprofits.com server is also in Canada, so that is not an issue.
See Also: Web Network Version Overview