What are the relations between environment , package and applications?

Package is an archive containing either MuranoPL classes or something (HOT is the only example at the moment) that can be converted to MuranoPL package. There are 2 types of MuranoPL packages: libraries and applications. The difference is that application packages contribute entry to applications catalog and library packages are not. This is analogous to executable/dynamically linked libraries separation in operating systems. Applications are classes inherited from io.murano.Application. Application packages expose one such class (for MuranoPL packages. HOT packages do that implicitly by a means of HOT package importer). From murano's perspecive environment is a class in core library (io.murano package) that is a container for applications (it has a property with a type "list of io.murano.Application") and other shared things like Heat stack, default instance networks etc. From a users perspective environment environments are minimal deployment units (you can deploy environment but not individual applications) that contains related applications linked together