Pioneer Era Postcards (1869 - 1898). "Plain" U.S. postal cards were inaugurated by the U.S. Postal Service in 1873, though one source says you can find cards back to 1861. (Printed messages, stamps, and cancellations make these much more valuable.)

Pioneer Era "View" Cards (1893 -1898) They started at the 1893 Columbia Exposition in Chicago.

Private Mailing Card Era (1898 - 1901). The backs are clearly labeled "Private Mailing Card."

Undivided Back (1901 - 1907) - no designated place for a message. U.S. Government mandated the words "Post Card" on the back.

Divided Back (1907 - 1915). From my experience it isn't always easy to distinguish between this era and the next with any sense of authority.

Early Modern or White Border Era (1915 - 1930). Backs are still divided but the images are poorer quality (due to the loss of access to German printers during the Great War). Reprinted images got white borders. The Detroit Publishing Company's photolithograph postcards fall into this era.

Linen Era (1930 - 1945). These might have white border, but they're easy to identify because of the good quality paper and linen finish. The colors glow.

Photochrome Era (1939 - present). Your ordinary glossy photo-offset postcards we are all accustomed to.

The 2 Buds also have illustrations of these eras.

Old Postcards.Com: glossary, quality ratings, and postcard history using the standard era definitions