cron are similar but different tools. They are both daemons, and normally you will install one or the other. They will each handle their own scheduled workload. If both are installed, neither should ever execute the others commands if both are installed.
cron is the older tool and designed for systems that are always on (servers). Newer versions will execute commands listed in
/etc/crontab. It is is possible to create crontab entries to run the
/etc/cron.monthly workload if anacron is not installed.
anacron is a newer tool better suited for systems that are not always on. It uses the file
/etc/anacrontab to control what should be run.