The anatomical neck is the one that looks like the neck from an
anatomy point of view-it is between the head (that goes into the
shoulder) and the tubercles.
The surgical neck is the site where most fractures occur. It
comes just after the tubercles. Remember that the humerus is kind
of in the shape of a "7" going into the shoulder joint, so the
anatomical neck is like if you cut from the top with a knife, and
the surgical neck is if you cut from the side.