1) A logical volume is NOT a (physical) disk partition. It is a virtual block device created inside Linux kernel with Device Mapper. It's a technology one can use to setup a single block device using two disk partitions as well as two block devices using a single disk partition, and much more.
# dmsetup targets
If you want to know more about DM visit it's home page, check it's manual pages, read RedHat manual or for instance this article. It's great technology only mentioned in Wikipedia.
2) For instance on my home computer I have physical disk partitions as well as DM devices (logical volumes).
# cat /proc/partitions
major minor #blocks name
3 0 80043264 hda
3 1 204800 hda1
3 2 79831029 hda2
3 64 19589976 hdb
3 65 104391 hdb1
3 66 19478812 hdb2
253 0 17432576 dm-0
253 1 2031616 dm-1
253 2 77762560 dm-2
253 3 2064384 dm-3
Primarily they don't have any type or ID, they are defined with just the start sector numer and the length.
# dmsetup status -j 253 -m 0
0 34865152 linear
So dm-0 is a fragment consisting of 34865152 sectors of some physical device.
# dmsetup deps -j 253 -m 0
1 dependencies : (3, 66)
And this device is /dev/hdb2 partition!
# fdisk /dev/hdb
Command (m for help): p
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 13 104391 83 Linux
/dev/hdb2 14 2438 19478812+ 8e Linux LVM
And it is LVM (8e) type partition, mentioned by timberman.
PV VG Fmt Attr PSize PFree
/dev/hda2 VolGroup lvm2 a- 76,13G 0
/dev/hdb2 VolGroup00 lvm2 a- 18,56G 0
This single (physical) disk partition can hold logical volumes within a group. I have two disks so there are two such groups and each group holds two logical devices.
LV VG Attr LSize Origin Snap% Move Log Copy% Convert
lv_root VolGroup -wi-a- 74,16G
lv_swap VolGroup -wi-a- 1,97G
LogVol00 VolGroup00 -wi-ao 16,62G
LogVol01 VolGroup00 -wi-ao 1,94G
So dm-0 and dm-1 are two LVs (lvroot and lvswap) created on my /dev/hdb2 partition of my first disk (with CentOS) while dm-2 and dm-3 are LVs created on my /dev/hda2 partition of my second disk (with Fedora).
Logical devices can have labels (UUIDs). You can check them with lvdisplay command, mentioned above. But they are just labels, which you can use (-u) instead of major, minor numbers (-j -m switches).