I put together a series of slides that takes you through the differences step by step.
The main purpose for the PPF is to illustrate the principle of Opportunity Cost when it comes to resource allocation. If an economy is at Full-employment to get more of one thing then something has to be given up.
Sometimes that trade-off may be "constant"--the resources taken away from the production of one good are "perfectly adaptable" to produce more of another good. A simple example is a farmer who has land where he can grow Corn and/or Soybeans. The land suitable for growing corn is the same as the land for growing soybeans (I live in Central Ohio--I see this just down the street). One the same acre of land, the farmer can get a maximum yield in corn or soybeans. Switching from one to the other entails virtually no cost in resource allocation for the farmer. How it affects society is another question.
However, if the crop mix is different and the resources used are NOT easily adaptable for a different use, then the opportunity costs are not constant but "increasing".
I use Corn and Rice as an example below. The land use for either is not identical. If I want to grow Corn where I once grew Rice then it may take 2 acres of rice field acreage in order to get corn yield equivalent to what I would get out of land perfectly suitable for corn production. My opportunity cost for more rice is not just one acre or rice production (Constant Cost) but two acres (Increasing Cost).
If the farmer persists in converting more of the rice field into corn production, then it may take 3 acres to get the equivalent in Corn. So on and so forth.
TINSTAAFL! Corn and Rice---now I am hungry. My opportunity cost of doing this blog entry is a delayed breakfast. You gave up eating lunch to read it. I hope it was worth it to you. Was for me. :)