These are committees that are part of a larger committee. 

Subcommittees are visible within the larger committee, and are a nice way to create a structured hierarchy.

 Subcommittees can be private or public, and each have access to their own area to define members, documents, emails and subcommittees. 

Members of a parent committee have access to information within the subcommittees, but members of a subcommittee do not get access to the parent committee information.