Just like pegmatite,
it is a term that describes a rock's texture rather than composition.
But unlike pegmatite,
porphyry does not only mean that the rock has large
crystals, it means it
crystals in a fine-grained matrix -
that's both large and very small crystals in the same rock.
crystals are called phenocrysts,
and they have formed before the matrix
that surrounds them.
Devil's Tower, a phonolite porphyry. By Lietmotiv via
Many rocks can be porphyritic, such as andesite porphyry,
rhyolite porphyry, basalt porphyry and many, many ohters.