There are two types of engineered wooden boards, plywood and blockboards.
Plywood is made from layers of wood veneers glued over one another, while blockboard consists of rectangular blocks of softwood covered on both sides by a thin slice of wood veneer.
So, what is Plyboard then, and is it the same as Plywood?
The manufacturers who make plywood also usually are in the business of making blockboards. The word 'Plyboard', I believe, is a combination of these two words used together. These companies mark their products (plywood or blockboards) by printing their company name and the word 'plyboard'.
The confusion is because of this naming convention followed. There are also other manufacturers who print their company name followed by 'Ply & Board', and yet others clearly mark their boards as 'Plywood and Blockboards'.
Take a look at the picture below. I found this piece of plywood lying around near the entrance of a garden in Navi Mumbai. Some carpenter had left his tools and a few boards over there.
And here is another picture. Notice the words 'Plywood, Block Board' written on the sheet. This type of naming convention is more widely followed and should mean that the company manufactures 'plywood' as well as 'blockboards', which are two distinct and separate things.
So according to me, the only difference between plyboard and plywood is that, plyboard is a generic term that is used to refer to a company that makes plywood as well as blockboards, while plywood is a specific type of engineered wood product.
Having said that, there are a lot of terms in the woodworking industry are used interchangeably. Here are some examples.
Plywood = Laminated wood
Blockboard = Block wood
Plyboard = Could mean either Plywood or Blockboards
Phenolic plywood = Film-faced Concrete shuttering plywood
and so on and so forth.