By selecting 'Send Announcements' in your auction listing, an email announcement will be sent out to users who have either active Bids or Ask similar to the item you are selling or who have had past Bids or Asks similar to the item you are selling. These users have selected to receive these announcements.

You can change your preference to whether or not you want to receive these announcements in the 'Edit Profile' section in the 'Your Control Panel'.