It's made from less expensive woods, like pine and poplar that are locally abundant. Better grades of plywood have a thin veneer (.050") glued on the primary visible side. Sometimes its on both sides. Better grades of plywood have better veneer to match hardwoods that may be used in framing and detail.
When they say Oak or Teak plywood they are referring to the veneer, not the internal wood.
Ans when they say 1/2" or 3/4" plywood they don't mean that either. However you can pretty much trust the sheet will be 4x8 foot when sold hat way.
But a 4x4 sheet and a 2x4 sheet won't be because they cut down 4x8 sheets for those and they lose a 1/8th inch or so for every cut.