netmeg - 12:31 am on May 13, 2010 (gmt 0) [edited by: netmeg at 12:32 am (utc) on May 13, 2010]
That's whatever the advertiser determines is a conversion. It can be a sale, a signup, a lead, even a pageview.
AdWords gives you special code to put on your confirmation page, so that when a conversion occurs, that code will track it.
[edited by: netmeg at 12:32 am (utc) on May 13, 2010]