Red Hat repositories description

Hello,

I was wondering if there were a description of the repositories provided by RedHat. Or even how a package get associated into a specific repository.

For example to find out the difference between rhel-7-server-optional-rpms, rhel-7-server-supplementary-rpms and rhel-7-server-extras-rpms.

I never know in which repo I could find what I am looking for.

Jeremy