Workflow
An org-wide automation engine: a Workflow rule watches for one of three trigger types (a file changing state, a task being completed, or a form being submitted) and takes an action -- send an email, assign a task -- when it fires, optionally only when a condition matches.

Note:
Workflow rules scoped to a specific form or module can also be created and managed from that object's own page -- see Form Overview's Workflow for the Form Submission case. Both entry points manage the exact same underlying rules; this page just isn't filtered to one object.