Clockworkmod Backup/Restore Feature So last week, I decided to flash a custom ROM to my 2 year old Desire HD. I used the all in one, one-step, "rooting" tool called AAHK (It doesn't just root the phone, it does a whole lot like S-OFF, Goldcard, etc).

My DHD is now running Jellytime R30. Everything is good.

Now, I would like to try other ROMS. I did a backup of my current system via Clockworkmod in Recovery. 

Question: Is this a full "image" snapshot? I mean, will I be able to restore the entire system  including the ROM when I restore this backup? Or do I have to flash the ROM in the snapshot before I restore the backup?

PS: I'm new to this custom ROM thing but I've done my fair amount of research. :)
What ClockworkMod does is a so called Nandroid Backup. Basically, this contains an image for each of the file systems of the device (i.e. one image per file system) -- and thus it is a full backup/"image snapshot". The format of the snapshot might differ and depend on the device and ClockworkMod version being used; for my Motorola Milestone 2 most of the file systems seem to be captured as YAFFS2 image, i.e. the image uses the same file system used on the device. So it is comparable with using dd on Unix/Linux (see Wikipedia), at least in my case.

Newer versions of ClockworkMod Recovery (6.0 and up) also do "incremental updates" to save space on the backup medium and allow for faster backup (see e.g. this TechieBuzz.COM article). For this, you might also want to check our question How can I tell CWM to perform a full backup instead of an incremental backup?

More details you can also find by following the clockworkmod tag.