Duotone is a halftone reproduction of an image using the superimposition of one contrasting colour halftone (traditionally black) over another color halftone. This is most often used to bring out middle tones and highlights of an image. The most common colors used are blue, yellow, browns and reds.
Now due to recent advances in technology, duotones, tritones, and quadtones can be easily created using image manipulation programs.
You can save money on your project by using just two spot colors. The main advantage of choosing duotone is that you only need two color plates, reducing the cost of printing. Also, by choosing the colors you use, you can match the colors you want exactly, such as Pantones or your company's signature color. The drawback is that your project will be limited to the colors that can be produced by mixing just two colors, so you will not be able to recreate full color images in duotone.
Duotone images are like CMYK images, but instead of using four pre-determined colors to reproduce your image, they use two colors of your choice. They are created by combining two grayscales of different colors, like the example above.