Registration ID
An ID issued by the C2DM servers to the Android application that
allows it to receive messages. Once the application has the
registration ID, it sends it to the third-party application server,
which uses it to identify each device that has registered to receive
messages for a given application. In other words, a registration ID is
tied to a particular application running on a particular device.
You can find the source of Registration ID here.
Device ID might be the unique identification ID of a particular device, as in IMEI/IMSI number or something of the sort.
AFAIK, both are needed to get C2DM working.