The Granny Knot resembles a Square Knot (reef knot) but with a second tie crossed incorrectly. When attempting to tie a square knot, it is easy to produce a granny knot accidentally.
One way to distinguish them is that in the Square Knot, each loop passes completely over, or completely under (not through) the neck of the other. The granny knot is an unstable knot and should be avoided as it will slip under load and can jam when tightened. It is recommended to tie a Square Knot in nearly all circumstances.