The basic difference is that a pentaprism is a solid block of glass. That said, it's pretty difficult. It has mirror material on all but two sides, so that the light enters on one side, is reflected on the inside and then exits on the other non-mirrored side. This means that only two air / glass interfaces are involved: one where light enters the prism and one where it leaves the prism. This reduces the loss of light to a minimum.
A penta mirror does pretty much the same thing, but it consists of several separate mirrors, so there is air in the middle instead of a solid glass block. This makes it much easier and less expensive. This also means that there is an air / glass interface for each individual mirror, so that the loss of light increases considerably.
As a result, a penta mirror is used almost exclusively in relatively simple cameras, where reduced cost and weight are more important than a relatively poor view through the viewfinder.