What is the term for rock formed from cooled lava at the surface?

Extrusive igneous rock, as opposed to intrusive, which cools underground - and the difference between them is entirely about speed, which you can see with the naked eye. Magma cooling slowly at depth grows large crystals, so granite is visibly speckled; lava cooling fast at the surface gives crystals no time to grow, so basalt is fine-grained and uniform.

Cool it fast enough and there are no crystals at all. Obsidian is volcanic glass, quenched so quickly that the atoms never organised, which is why it fractures into edges sharper than surgical steel - a few molecules wide. It was traded across enormous distances in the ancient world for exactly that, and obsidian scalpels are still used in some specialist surgery.

The two also produce completely different landscapes. Runny basalt lava builds the broad shallow shields of Hawaii and the columns of the Giant's Causeway, which form as a cooling flow contracts and cracks into hexagons. Sticky silica-rich lava barely flows, piles into a dome, and eventually removes the top of the mountain - which is why the least dramatic-looking volcanoes are usually the dangerous ones.