Fiends & Machines was part of the 'car-and-monster' craze of the late 60's and early 70's, which always paired a horrible (but somehow humorous) creature or character with a cartoony, supercharged automobile. Odd Rods was probably the ultimate expression of this in the card collecting hobby. The trend started in the first place with the artwork of Ed 'Big Daddy' Roth.
The stickers were separated into two groups of 33 each: #'s 1-33 are the Fiends, while #34-66 are the Machines. The Fiends are mostly monster heads which line up with barely-protruding necks at the tops of the Machines cards, so that when the one is placed on top of the other, the two cards make one image of a monstrous visage arising out of a souped-up car. A humorous slogan is included in the image, continued from the top card to the bottom (and open-ended enough so that any combination of tops and bottoms will work together, more or less).
These being stickers, the backs were merely instructions on how to peel and apply them.