Sling Bundle Project: This project contains your Java code. This is were you can add your OSGi services, models etc.
Sling Content Project: Content projects contain the content you want to deploy to your JCR repository (which is part of Sling). Content in this case mostly means XML files which describe the content structure that is added to the repository. But it can also contain JavaScript and CSS files or images.
Note that these are two templates provided by the Sling IDE Tooling for Eclipse and intended as starting points.
Sling content projectandSling bundle project? - Jens