The wet-on-dry technique is when your paper is dry and you lay down your watercolor on dry paper. The paint does not flow around the paper like it does when the paper is wet. Brushing paint on your dry paper will only remain within the shape you defined with your brush. This produces hard edges and is called the wet-on-dry technique.
There is more control over your brush strokes when you apply paint in this manner. This is because the paint will not flow beyond the wet borders you made with your brush stroke.

This technique is useful for adding details in layers. Commonly seen in realistic style watercolor.
Wet on dry is the watercolor painting technique you use when you want to have hard edges and detail. This will allow you to build up details and hard edges and adds values and vibrancy to colors with additional layers.
You can also use it to cover an area with a saturated wash without the dilution of water on the paper. This will produce a much more vibrant color. To get this effect you must have enough of your paint premixed in order to get a smooth wash and not run out of paint. Premix puddles of paint before attempting this technique.

Wet on dry glazes
The wet-on-dry technique is brilliant for making glazes of transparent color for creating layers and depth. Think stained glass!! This is the most basic and beautiful technique for beginners. What a magical and lovely sight!

Dry on Dry technique
Also known as dry brushing technique. As its name suggests, use only the minimum amount of paint on your brush so that it is basically "dry" I achieve this by dabbing my brush on a piece of paper towel first to make sure it is adequately dry. You then lightly brush it over the paper and the pigment will get deposited in broken strokes. Using cold-pressed, or rough paper is best for this technique because the toothiness of the paper really picks up the texture effects of dry brushing.

Can you guess what the hardest part of the wet-on-dry technique is???
Waiting for your paper to dry!! LOL Walk Away! Grab a cup of coffee or tea and let it dry.