dm-0 and friends (dm-1, dm-2, etc) are devices used by Device Mapper when LVM is in use.
To see what LVM volume each one relates to, run the following command (all on one line).

sudo lvdisplay|awk '/LV Name/{n=$3} /Block device/{d=$3; sub(".*:","dm-",d); print d,n;}'

Source: http://superuser.com/questions/131519/what-is-this-dm-0-device