Pressed Wood is a broad category that includes Particle Board, OSB, MDF, and the trademarked Masonite [edit: Masonite is a type of "hardboard], as well as, engineered wood (think: engineered flooring), particle board, and plywood as these all require the product to be pressed (compressed).
Particle Board is a pressed wood that includes OSB and, maybe, Masonite as both are made of wood particles/fibers that are not made of thin sheets of wood, paper, or pulp. Usually, when people say, "particle board" they're referring to the type used on ready-to-assemble (RTA) furniture. It can be identified by looking at the cut ends where one can see wood particles, as well as, voids. This product is almost always laminated.
OSB (Oriented Strand Board) looks like a patchwork of thin scraps of wood and is used for house underlayment (though I prefer plywood for this). It is never recommended for applications where it would get wet frequently as it will come apart after a while (of continuous wetness).
Masonite is smooth on one side and a pattern of diamonds or squares on the other. It is most often used as the backing of ready-to-assemble (RTA) cabinets and the backing of factory made furniture (ie. end tables, etc.). Older clip-boards are also made of it. Peg-board is the same/similar material.
MDF is made of several sheets of paper that have been pressed together (get it wet and these layers will separate from each other....permanently). MDF is a favourite for chair rails and crown molding and the like.