What is the difference between iPhone 6, 6s and 6+?

The iPhone 6 and 6 Plus are the same generation using the same CPU and other hardware, the + has a bigger screen.

The 6s is the next generation after the 6, has a more powerful CPU, better camera and more RAM.

There's a 6, a 6 Plus, and a 6s and 6s Plus.

As deggie has already mentioned the difference between the regular model and the plus model is screen size and larger battery primarily.

The 6s however is heavily improved over the 6.

Compare them here: https://www.apple.com/iphone/compare/

You need to select both 6 and 6s in the menu.