Lava flows are typically finer grained (i. e. the rock is made up of smaller mineral grains) than intrusive igneous rocks because the extrusive magma cools quickly so the mineral grains do not have time to grow.
When lava comes out of a volcano and solidifies into extrusive igneous rock, the rock cools very quickly. Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. Magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture.
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