As for why they are called loop devices - that's something unsettled. In UNIX Stack Exchange, user schily said that he created the feature first on SunOS 4.0, which he called fbk (file emulates block device); and he complains that Linux seems to use a wrong name for that feature.

According to others, loop is short for "loopback" - and some say it was chosen to show how the block devices are looped back. I don't know what it means. Solaris 8 introduced lofi (loopback file) which worked similarly, and BSD introduced them under the name vnd (vnode disk).

So losetup can be used to create a loop device as /dev/loopN based on a disk image. More precisely, a block device is any device in /dev. A loop device is a block device for a disk image file.

And only loop devices are mounted using mount. When you use mount on a disk image, thought it seems direct, it actually runs losetup under the hood. In fact, the option -o loop does exactly that.

If the disk image contains many partitions, you cannot mount it as a single file system. You you have to inspect the partition table, using a utility like fdisk or gdisk, and then create loop devices for the specific offsets of the image, and mount them.

kpartx from util-linux used to do that automatically, and now losetup has an option that does that as well.

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