Once, not long ago, I thought that porcelain tile and ceramic tile were pretty much the same thing. What a dummkopf, huh?
Oh wait: they are. Retract that self-criticism.
According to Angela Miller, of mega tile manufacturer Dal-Tile, porcelain and ceramic are both members of the same, larger ceramic tile family. Except the sub-category that we call porcelain tile is harder, stronger, and more expensive than its softer, cheaper cousin, which we call ceramic tile.
Porcelain tile has a higher PEI hardness rating (think: Janka wood rating, but for tile). But there is one anchor in this debate you should keep in mind: porcelain tile's color runs from top to bottom. Ceramic tile's color is baked onto the top.
So, if you chip porcelain tile, the chip is barely noticed. If you chip ceramic tile, you chip the top layer--thus, a different color shows through.
Image: Copyright Lee Wallender; Licensed to About.com