what are the differences among these two types of tiles?

It's d same.

Homogeneous - the tile has the same colour and material from the surface to the bottom, thus when the surface chips, the middle part still has the same colour. Usually it is a porcelein tiles.

Porcelein tiles, it is a stronger type of tiles compared to ceramic tiles. Basically the same material as ceramic tiles butit is produced under a hotter process thus more "dense and solid". Can be homogeneous or non homogeneous.

This is what I understand, i might be wrong and you can go through this thread on that has been discussed so far...
http://forum.lowyat.net/topic/1761694