In order for quotas to be effective, they must be enabled in your kernel configuration and enabled at boot time, usually by an init script which calls the quotaon command on each filesystem that is to have quotas. It is also common to run a quotacheck utility before enabling quotas which tallies up the block and file usage for each user who has files in the filesystem and initializes the quota files (usually some special file in the filesystem root) with those usages. Once quotaon is run, the operating system tracks all file usage for all users who create or modify files in the filesystem and updates the quota files, also preventing file creation or writing that would exceed a user's configured quota. quotaoff turns off quota handling in a filesystem (sometimes done on shutdown).
The edquota command is the typical manual user interface for modifying a single user's quota; it may also have modes for batch setting of quotas by copying one user's quota limits to another's. When manually editing a user's quota, it presents a human-readable description of the user's quota settings in an editor; on exit, it parses the quota description and modifies the user's settings.
The quota command allows a user to inquire about his or her quota settings; normally it just reports any over-quota status, but if invoked with quota -v it reports the user's current quota limits and allocation.