When you are using EBS-backed instances, the Cloud is going to create a new volumen (vol-ID) which is going to be used as your root file system.
This is useful is the case that you have to store data (persistence) and also to allow to back it up (with snapshots).
On termination these volumes created will remain available, which can be a problem as it will consume disk space, or it will take you time to delete them all.
To avoid this, you can register your EMI adding the "delete on termination" flag
For example :
euca-register -n <Name> -d <Description> -b /dev/sda=<snapshot-id>:size:<delete on termination:true/false> --root-device-name /dev/sda
Set True for the last argument will enable delete on termination, this way you do not have to delete all volumes by hand.