How to put different textures on each side of a plane.

If you have a plane in Blender, you can put different image textures on each side or each face of the plane. This feature is helpful in case you want to model a banknote that has different text/images on each face as shown in the following image, or similar applications. 

In the Shader Editor, The Backfacing output of the Geometry node gives a value of 0 for the front face of a plane, and 1 value for the back face.  

So, you can use a node setup like that to implement different images on both sides. We use a MixRGB node and it has a factor input and two input colors. If you connect the backfacing output to the input factor of the MixRGB node, it'll separate the two colors (and separate the two image textures).