Front and back post double crochets
As the name suggests, the front post double crochet and back post double crochet (abbreviated fpdc and bpdc, respectively) are worked around the post of a stitch rather than into the space created by a stitch in a previous row or round. These stitches are great for adding texture to a crocheted piece and there are some stitch patterns that use combinations of these 2 stitches. For example, to get a ribbed knit look in crochet, you can alternate front and back post double crochet stitches along a row. You can also get a variety of different basketweave looks by doing different combinations of sets of stitches and rows. Front and back post stitches do not have to be added to the stitch below but can be added in other places depending on how big the post stitch is. This can be used to give crochet 'cables'. The photo below shows 2 different examples of the use of post stitches (some of the post stitches below are fptr rather than fpdc). The photo below shows some cables along the side of the button band and a ribbed look at the bottom.
Front post double crochet
To do a front post double crochet, yarn over, insert your hook from front to back around the post of the stitch and pull up a loop. Yarn over and pull through 2 loops, yarn over and pull through 2 loops.
Back post double crochet
Yarn over, insert your hook from back to front around the post of the stitch. Yarn over and pull through 2 loops, yarn over and pull through 2 loops.