Composition (mineralogy) refers to the relative amounts of silica vs. mafic minerals. High silica rocks (silica > 60%) are considered felsic. These rocks, like granite and rhyolite are generally light in color. Rocks rich (>60%) in ferromagnesian minerals (mafic) are generally dark in color.
Texture describes the nature of the grains (crystals) that make up the rock. Rocks are considered coarse grainded if you can distinguish the crystals with the unaided eye. Fine grained igneous rocks have at least a portion of the rock matrix that has no visible crystals to the unaided eye. Porphyritic texture is produced by two distinct cooling stages producing both large and small crystals in the same rock. Slow cooling (generally deep underground) produces large crystals. Rapid cooling (at or near the surface of the Earth) produces smaller crystals. In a porphyry, the crystals are of distinctly different size. The smaller crystals are called the matrix or groundmass. The term pegmatite is reserved for igneous rocks that have unusually large crystals. It is a term that is used for any such igneous rock but is normally associated with granites. Pegmatites are unique in that they do not form directly from the igneous melt but instead are formed from fluids derived from or near the igneous rock body. The fluids (generally aqueous and under high temperatures and pressures) allow for a great deal of freedom for the migration of ions (charged atoms or molecules) to the sites of crystallization. The result is the formation of large crystals.