"external" is a group of modules that while not required for storm to operate, are useful for extending Storm in order to provide additional functionality such as integration with other technologies frequently used in combination with Storm.
External modules are released in tandem with Storm in order to maintain version compatibility.
Each external module is assigned one or more "Committer Sponsors" who have expressed interest in supporting the module.
A Committer Sponsor is simply an Apache Storm Committer who has expressed interest in supporting a module in one way or another. For any given module, we hope to have at least one sponsor to provide some level of protection against code rot and abandonware.
Note that this in no way implies that Committer Sponsors have any special roles, privileges, or obligations. The Apache Storm Committers have equal authority over and responsibility for the entire codebase. A Committer Sponsor is essentially just a Committer who has said: "I'm interested in this module, and am willing to help where I can."