What it is:
    Anacron 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.

What it is not:
    Anacron is not an attempt to make cron redundant.  It cannot
    be used to schedule commands at intervals smaller than
    days.  It also does not guarantee that the commands will be
    executed at any specific day or hour.  It isn't a full-time
    daemon. It has to be executed from boot scripts, from
    cron-jobs, or explicitly.
