What is the difference between dandruff ahd seborrheic dermatitis? I have large flakes in certain areas of my scalp, especially around the edges.

Same. dandruff refersto the flaking part of seborrhea ( Oily scalp condition) and seborrheic drmatitis is seborrhea with skin ( scalp) inflammation. Easily treated with most oTC dandruff meds.
Seb derm. Mild seborrheic dermatitis and dandruff can essentially appear the same, but seb derm is often associated with redness, scaling, and crusting. Seborrhea also can affect other parts of the body, including the face, chest and upper back.