1. Domain projects in Mule allow resources like connectors to be shared across multiple applications. Each Mule project is associated with a domain by default. 2. Common configurations can be defined under a domain project and then used by Mule projects. Domain projects are deployed to the mule/domains folder rather than the mule/apps folder. 3. Domain projects do not support minor version changes, so a domain must always be deployed with the same major version number even if the content has changed.