There are several general types of igneous rock texture. The texture results from the cooling history of the rock. Mineral crystals take time to grow - if magma cools slowly, large crystals can form. Lava generally cools faster, so the minerals are generally small in extrusive rocks. They are intrusives that cool slowly inside Earth.
Porphyritic rocks are a combination of the two above.
These rocks contain a fine matrix with no discernible crystals and embedded phenocrysts, large isolated crystals that are easy to spot. Pyroclastic rocks are formed by the explosive ejection of magma from volcanoes and rapid cooling. Such rocks often have vesicles and bubble-like features. Glassy rocks also form when magma is quickly quenched. They have no crystalline structure and break in sharp edges.