What are Managed and Unmanaged packages in salesforce?

Unmanaged packages are typically used to distribute open-source projects or application templates to provide developers with the basic building blocks for an application , whereas Managed packages are typically used by salesforce.com partners to distribute and sell applications to customers.

Main thing which differentiate Managed and Unmanaged is , Managed version can be upgraded but the Unmanaged version you can customize however you like, but it can't be upgraded (if a new version were released, you would need to uninstall and reinstall, which can be difficult once you have data). That is the reason , unmanaged package is only used to share the meta data.