cgroups.7: Add a terminology subsection

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/cgroups.7

@@ -71,9 +71,35 @@ cgroup which is used by
 .BR systemd (1)
 to track services and user sessions.
 .\"
+.SS Terminology
+A
+.I cgroup
+is a collection of processes that are bound to a set of
+limits or parameters defined via the cgroup filesystem.
+
+A
+.I subsystem
+is a kernel component that modifies the behavior of
+the processes in a cgroup.
+Various subsystems have been implemented, making it possible to do things
+such as limiting the amount of CPU time and memory available to a cgroup,
+accounting for the CPU time used by a cgroup,
+and freezing and resuming execution of the processes in a cgroup.
+Subsystems are sometimes also known as
+.IR "resource controllers"
+(or simply, controllers).
+
+The cgroups for a subsystem are arranged in a
+.IR hierarchy .
+This hierarchy is defined by creating, removing, and
+renaming subdirectories within the cgroup filesystem.
+At each level of the hierarchy, attributes (e.g., limits) can be defined;
+these attributes may govern or propagate
+to child cgroups and and their descendants in the hierarchy.
+.\"
 .SS Cgroups version 1 and version 2
 The initial release of the cgroups implementation was in Linux 2.6.24.
-Over time, various cgroup subsystems (controllers) have been added
+Over time, various cgroup subsystems have been added
 to allow the management of various types of resources.
 However, the development of these subsystems was largely uncoordinated,
 with the result that many inconsistencies arose between subsystems