In simple terms GPWS just looks at you radio altimiter to dertermine if you are about to bump into anything solid, which is useless if you are flying into a vertical cliff sticking out of level ground.
The EGPWS system incorporates a GPS so that the aircraft knows exactly where you are, it also knows the location of terrain all over the world, and so is able to work out if you are going to bump into anything on your route. A much safer system than simply relying on a radio altimiter!