throughout my work on cgroups I have had many moments in which I look at a struct definition or variable declaration or even a function call and have nothing to think but "uhhhhh??" there is a nontrivial amount of infrastructure in the cgroup world, and here I am going to attempt to do a quick run-through of how everything is set up.
struct cgroupfs_root: this represents the root of a cgroup hierarchy. it knows things like what subsystems are attached to it, the root cgroup in the hierarchy, and other trivialities like its own name. variables of this type are almost always called "root".
struct cgroup: represents a single cgroup. remember that each directory, looking through the VFS layer, is a cgroup, so when you 'mkdir cgroup/foo', a new cgroup is created. variables of this type can be seen most commonly as "cgrp", but also sometimes "cg" or simply "c".
struct cgroup_subsys: the heart of the subsystem API - function pointers for the subsystem's operations, and other various options. referred to usually simply as "ss".
struct cgroup_subsys_state: per-cgroup, per-subsystem state storage - when you write to a subsystem control file for a given cgroup, this is what hears about it. always called "css".
struct css_set: a collection of pointers to cgroup_subsys_state objects. each css_set is referenced by all tasks who use the given combination of subsystem states. any particular subsystem state for a given task is only likely to change if the task gets moved into a different cgroup in the hierarchy to which the subsystem is attached. the presence of these guys is purely an optimization, since multiple tasks can have the same combination of subsystem states, and will therefore use the same css_set. additionally, they are stored in a hashtable for quick access. in comments, this data structure is referred to as a "cgroup group", and variables tend to go by the name "cg", but sometimes "css" as well.
struct cg_cgroup_link: as per the comment, this is a "link structure for associating css_set objects with cgroups". each of these has a pointer to one struct cgroup and to one struct css_set, and lives at the intersection of two lists: the first list, per cgroup, links all css_sets associated with that cgroup; the second, per css_set, links all cgroups associated with that css_set - forming, as is described in Documentation/cgroups/cgroups.txt, a "lattice". they are what you want to use if you want to iterate either all css_sets in a cgroup or all cgroups in a css_set, and respond mostly to "link" but sometimes "cgl".