The stitch that is the core of a granny square and makes it recognizable as such is the so called "granny stripe pattern". The granny stripe consists of sets of three double crochet stitches filling chain spaces across and around. And thus they make up a traditional granny square. The center of such a granny is most often a 2D flower/clover or a circle.
However, there are other types of grannies: lace / mesh squares, with or without (3D) flowers, diagonal (corner to corner), vertical, mitered and the list goes on.
Beside squares, there are the granny triangles and hexies, which add up to and make the crochet world even richer and more beautiful.
What's your favourite type of granny?