It seems to me that maybe these names for flooring tile seem to be used interchangeably. What is the difference in these two types of tiles and which is better to use on a garage floor, and why?
This is somewhat incorrect. While porcelain in available in through body, less than 5% of porcelain is through body.
The most desireable feature of porcelain is it is much denser than ceramic and because of this, absorbs much less moisture than ceramic leading to a more frost resistant tile. Hence it's primary use was meant for outdoors and commercial applications.
There are ceramic tiles available that are just as "strong" as some porcelains but overall, porcelain typically gets the nod for being stronger/harder.