Technically Sonic'06 would be a UDF image since it's on a DVD, and DVDs use the UDF filesystem instead of ISO-9660.
>_>
*shot*
ROM image = a bit-for-bit copy of the data stored on a ROM chip, which is why a dump of a ROM cartridge is called a ROM.
ISO image = a copy of the data track from a CD-ROM, usually in ISO-9660 format (but may also have MS Joliet or UNIX Rock Ridge data).
UDF image = a copy of the data track from a DVD-ROM, usually in UDF format. (Sometimes mistakenly called ISO because it's from the same type of medium.)
Also note that not all CDs are ISO-9660 (though most are), and not all DVDs are UDF. Some DVDs are combo ISO-9660/UDF for compatibility, while some are just plain ISO-9660. Also note that DVDs cannot contain CD audio tracks (for obvious reasons), while CDs may contain both an ISO-9660 data track and multiple audio tracks.