The purple part of this sock is the gusset.© Sarah E. White, licensed to About.com, Inc.
The gusset of a sock is a sort of triangular-shaped piece of knitting that connects the stitches that were last worked for the leg of the sock to the stitches that were used to work the heel flap and to turn the heel.
The gusset is formed by picking up stitches along the heel flap and gradually decreasing until you get back to the number of stitches you had when you cast on.
At the same time that the gusset is knit, the leg stitches continue onto the top of the foot, and any design that has been knit into the leg can continue on these stitches, if desired.
In the picture at right, the gusset is the part knit in purple.