Some times you need a new filesystem, for testing purposes or for whatever and you dont have access to a physical device. You can create a filesystem within a file as follows:
dd if=/dev/zero of=myfsys bs=512M count=1 # creating a 512M zeros file
mke2fs myfilesys # creating a file system out of the file
mount myfilesys /mnt -o loop=/dev/loop0 # mounting it as a loop device
mkdir /mnt/point1 # creating a mount point
mount /dev/loop0 /mnt/point1 # mounting
df /mnt/point1/ # its alive!
# results with:
# Filesystem 1K-blocks Used Available Use% Mounted on
# /dev/loop0 516040 400 489428 1% /mnt/point1