Tintypes are a variation of the collodion wet plate process.  The emulsion is painted onto a japanned (varnished) iron plate, which is exposed in the camera. 

Daguerreotypes, ambrotypes and tintypes were one of a kind images, and the image was almost always reversed left to right.

The low cost and durability of tintypes, coupled with the growing number of traveling photographers, enhanced the tintype’s popularity.