in the kernel source code there is a documentation folder... find the file relating to devfs and read it
basically, the tty's are for input (usually by keyboard)
and the loop allows you to use files on hard disks, asif they were hard disks themselvs..
make a 50 megabyte file
dd if=/dev/urandom of=/bigfile
make a loop device
losetup /dev/loop1 /bigfile
now, /loop1 can be treated as a seperate hard disk... the hard disk will be saved as /bigfile
and can be formated mounted, or whatever you like.
i use the loop to make a seperate home parttion.