Are these two terms interchangeable, or is one a subset of the other?
Managed Metadata service is a service offered by sharepoint . The MMS is divided further in taxanomies(term store),folksonomies(keywords),syndicated content types .
The managed metadata service application has two purposes, 1. To make it possible to use managed metadata and 2. Share content types. The MMS has 1 term store metadata service applications and cant share the term stores. The term store is subset of MMS .