1. Anacron (like `anac(h)ronistic') is a periodic command scheduler. It executes commands at intervals specified in days. Unlike cron, it does not assume that the system is running continuously. It can therefore be used to control the execution of daily, weekly, and monthly jobs (or anything with a period of n days), on systems that do not run 24 hours a day. When installed and configured properly, Anacron will make sure that the commands are run at the specifiedintervals as closely as machine-uptime permits. This package is pre-configured to execute the daily jobs of the RedHat Linux system. You should install this program if your system is not powered on 24 hours a day to make sure the maintenance jobs ofother Red Hat Linux packages are executed periodically. From Redhat 8.0 RPM

2. a cron-like program that doesn't go by time Anacron (like `anac(h)ronistic') is a periodic command scheduler. It executes commands at intervals specified in days. Unlike cron, it does not assume that the system is running continuously. It can therefore be used to control the execution of daily, weekly and monthly jobs (or anything with a period of n days), on systems that don't run 24 hours a day. When installed and configured properly, Anacron will make sure that the commands are run at the specified intervals as closely as machine-uptime permits. This package is pre-configured to execute the daily jobs of the Debian system. You should install this program if your system isn't powered on 24 hours a day to make sure the maintenance jobs of other Debian packages are executed each day. From Debian 3.0r0 APT