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.

One of the new features in CRM 2011 is called Solutions. This feature allows developers to group customizations into predefined packages and import/export these to different organizations. This basically is an improvement of the CRM 4.0 customization import/export. On top of simple grouped import/export solutions can be defined as managed, limiting the abilities to change the solution once import into a different organization. A managed solution can also be deleted.