What is the meaning of ps process names printed in [] such as [cgroup]?

What is the meaning of ps process names printed in [] such as [cgroup]?

when I do a command such as ps -aux on CentOS 6 I get a bunch of processes
whos command is listed in [] as shown below. What is the meaning of the []
in the name? I am assuming that these are special processes of some kind,
what makes a process name show up with a [] around it?
[root@centos6 src]# ps -ef
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 18:48 ? 00:00:01 /sbin/init
root 2 0 0 18:48 ? 00:00:00 [kthreadd]
root 3 2 0 18:48 ? 00:00:00 [migration/0]
root 4 2 0 18:48 ? 00:00:00 [ksoftirqd/0]
root 5 2 0 18:48 ? 00:00:00 [migration/0]
root 6 2 0 18:48 ? 00:00:00 [watchdog/0]
root 7 2 0 18:48 ? 00:00:02 [events/0]