Satin stitches are made of straight, parallel side-by-side stitches that solidly fill in an area. It it often used for flowers (both petals and leaves), bricks in a house, and other small areas that need dense filling. The stitches should be worked close to each other, and not be too long or they will not lie well on the fabric. Satin stitches are also a basic component of kloster blocks in hardanger.
A closeup of the Satin Stitch:
Check out this tutorial on the Satin Stitch from Namaste Embroidery on Youtube: