Clockwork is a cron replacement. It runs as a lightweight, long-running Ruby process which sits alongside your web processes (Mongrel/Thin) and your worker processes (DJ/Resque/Minion/Stalker) to schedule recurring work at particular times or dates.

For example, refreshing feeds on an hourly basis, or send reminder emails on a nightly basis, or generating invoices once a month on the 1st.