These are called hard exudates. They are the lipid residues of serous leakage from damaged capillaries. The commonest cause is diabetes. Other causes are retinal vein occlusion, angiomas (Von Hippel-Lindau Disease), other vascular dysplasias, and radiation-induced retinal vasculopathy.
If you think you could confuse hard exudates with other yellow-white things in the retina (yes, easily), check out Yellow-White Things in the Retina.