ulimit is a built-in command of the bash shell. It is used to control the resources available to a bash shell and all the programs that are started from this shell. Thus, it can be used by the system administrator to control the resources available to the users by setting the ulimit in the file /etc/profile.
However, a better way of limiting user resources is through the pam_limits(8) module. See manlimits.conf, man pam_limits. We are leaving here the beginner's playground, though.