How secure is CiviCRM?
CiviCRM does not do anything special to either keep your data secure or insecure. It stores data in a MySQL database like lots of other applications.

The security is more in the way you configure your database server and the way CiviCRM (which is a web application and not a desktop application) can be accessed from outside. 

A little more information in the configuration we tend to advise as the most secure (but also as usual most cumbersome and expensive) can be found in this CiviCon session by my community mate Björn Endres: https://www.youtube.com/watch?v=JKNR0bDjB9k