They're the same thing in so far as they are both worn overtop of a diaper. They are different when it comes to design.
Plastic pants are what they sound like, a soft plastic pant that you pull overtop your diaper, they are cut and designed more like shorts or bloomers I guess. They can have cartoony designs or they can be just plain coloured plastic.
A diaper cover is usually more ornate, some of them have snaps and velcroes. They are more form-fitting, like an extra protective covering for the diaper. They can also be made from a variety of materials, not just plastics. I own some that are soft and poofy and some that are more firm.
I own both, but I prefer diaper covers, they contour to the diaper better and they can be extremely babyish courtesy of some of their designs and the fact that some of them have snaps.
I found some links that should demonstrate the slight differences between the two:
http://abdlfactory.com/image/cache/...LT Diapers Plastic Pants Cover2-1000x1000.jpg
Diaper Cover (a particular type of one anyhow)
https://www.plastic-pants.com/images/DeluxePant/DeluxeNewA.jpg
Plastic Pants (a fairly standard representation)