This is a CDV (Carte de viste) photo of two men sitting in chairs. It was taken by an A. McCormick, of Oxford, Chester County, Pennsylvania. I can date this to the 1860s for a couple of reasons, but mainly because there is a green George Washington 3 cent tax stamp on back. I believe this particular stamp dates this photo to 1864.
CDVs are small, usually about 2.5 x 4 inches, usually with an albumen print affixed to a cardboard backing. They were popular from the 1850s to the 1880s at least. The thickness and look of the cardboard, plus the styles of the graphics advertising the photographer also give clues to the date. If you're expert enough, you may also be able to tell by clothing or hair styles - of course people aren't always current with the latest fashions, especially older people. Sometimes people are nice and write names and dates on it, but usually not. And sometimes it has a tax stamp.
I like this CDV because it has two men in it, they have interesting mustaches and beards, the hats seem a little unusual, and they're wearing long coats, they both have on what looks like the same kinds of boots, and of course, the tax stamp on back. And the actual picture looks nicer than what I could reproduce.