The texture of volcanic rock is related to the length of time it took to cool and harden. Rock that cools more slowly has larger crystals, as it has more time to grow. Granite is an example. Granite is an intrusive igneous rock, meaning it forms beneath the earth's surface. This insulates it from rapid temperature changes and allows it to cool more slowly.
Rock that cools quickly has finer crystals that often aren't visible to the naked eye. Basalt is an example. It's an extrusive igneous rock, meaning that it cools at the earth's surface. It's not insulated from the atmosphere so it cools to the ambient temperature quickly.
Obsidian cools even more quickly than basalt and has little time for crystal growth. Obsidian is also called volcanic glass. It's an extrusive rock that often forms at the edge of a lava flow where cooling takes place faster.