Since the late 90's Wheel Skins have been slowly introduced on a select models and over the years many have not taken much notice and so often these are mistaken for each other.
What are wheel covers?
Wheel Covers are a decorative covering that is placed over the STANDARD BASE STEEL WHEELS of a vehicle. These tend to be silver or black and have multiple holes on the outer edge and without a doubt tend to be UGLY and need to be covered. Most of these covers will be snapped on and use clips and a ring. There are a few occasions in which the covers are bolted on under the factory lugs or spin onto the factory lugs.
Wheel Skins are also decorative coverings however they are EITHER placed over STYLED STEEL WHEELS or ALLOY WHEELS. The majority of them simply snap onto the wheels with little effort. Some will require you to remove the center cap emblem and snap into the wheel skins for a factory finish look.
WHEEL SKIN EXAMPLE:
You can see why the REQUIRED wheel is necessary as the design is perfect fit:
Wheel Cover or Hubcap Example
STEEL WHEELS that just use Hubcaps or Wheel Covers (NEVER WHEEL SKINS)