Granite is a natural product whose patterns will vary with each slab. It is extremely durable, being highly scratch resistant and heat resistant. Granite is also a fairly porous material that should be treated with a sealer to ensure its resistance to stains.
Quartz is a man-made product consisting of quartz and bonding agents which make it very strong and non-porous. As a man-made material, the slabs will have a more uniform pattern. Quartz does not need to be sealed.
Corian is a man-made product consisting of acrylic polymer and minerals, which imitates the look of granite and quartz countertops. It has a uniform appearance with almost invisible seams. It is non-porous and resists stains. Corian can show scratches but is usually easily repaired because the color runs all the way through the thickness of the top.