The crontab is a list of commands that you want to run on a regular schedule, and also the name of the command used to manage that list. Crontab stands for “cron table,” because it uses the job scheduler cron to execute tasks.
First, basic terminology
cron(8) is the daemon that executes scheduled commands.
crontab(1) is the program used to modify user crontab(5) files.
crontab(5) is a per user file that contains instructions for cron(8).
Every user on a system may have their own crontab file. The location of the root and user crontab files are system dependant but they are generally in /var/spool/cron/ directory.
System-wide crontab file
/etc/crontab file, the /etc/cron.d/ directory may contain crontab fragments which are also read and actioned by cron.
Linux distributions also have /etc/cron.* directory, scripts in this location will be executed by root privileges
/etc/cron.hourly/ — scripts which are placed in that directory will be executed by hourly
/etc/cron.daily/ — Scripts which are placed in this path will be executed by daily
/etc/cron.monthly/ — Scripts will be executed by monthly
/etc/cron.weekly/ — Scripts will be executed by weekly