dojo._base.event contains functions to handle DOM events.
As with all dojo._base components, these functions are included within Dojo Base. You get this functionality by just including dojo.js or dojo.xd.js in your page.
See Dojo's Event System for information on the event system.
Since V0.9 most of the functionality from the dojo.event package has moved to dojo base so it is always available. Users of older releases will find more details within the Dojo Porting Guide: 0.4x to 0.9.x.