A satin stitch is a series of flat stitches that are used to completely cover a section
of the background fabric.
These stitches are laid from side to side. A satin stitch is often used to outline and attach an appliqué to the ground fabric.
Running stitches are simply used to connect sewing areas in the design and for thin outlines or borders and areas of detail when a satin stitch is too large to be used.
Running stitches are only as wide as 1 strand of thread.
Fill stitches are used to fill large areas with color
Simplified…they are simply large areas of running stitches that are placed side by side to cover a large area.