Crochet overlay is a form of crochet that gives you a nice dense cloth, while Lace crochet is just the opposite, it has lots of open spaces, look at this tutorial from Lilla Bjorn:
The stitches are all basic: slip stitch and a double treble stitch, this type of crochet takes a little more time because you work to fill up all open spaces.
Another stitch it uses often is the front post double crochet, and back loop double crochet, these two stitches gives your project a nice 3D effect.
Enjoy!!! Have a Happy Tuesday, It is A Gift!