In CRM 2011 there are two approaches you can use to create solutions; Managed and unmanaged. There is lots of information available as to what the differences are, for example on channel 9. In general I would recommend using managed solutions every time you are exporting customizations from one environment to the next (in a dev, test, prod environment). But this article specifically addresses the development of applications/products using multiple solutions. I have seen various approached used and people running into problems caused by developing multiple unmanaged solutions in one organization.