Igneous rock was formed when molten rock solidified as it cooled. When magma solidified deep underground, it is called an intrusive or plutonic igneous rock. If the magma came to the surface (in which case it is called lava) before solidifying, it is called an extrusive igneous rock.
The size of crystals in an igneous rock is determined by the cooling rate. Extrusive rocks cooled quicly and therefore form a more homogenous mass of small crystals (visible under a microscope) whereas the slow cooling rate of intrusive rocks allowed sufficient time for larger crystals to grow.
Volcanic glasses are amorphous, meaning that they did not crystallize during formation. One possible mechanism for this is the rapid cooling that takes place when lava comes into contact with cold water. Thus their presence on Mars would be a possible, but not certain, indication that liquid, probably water, was present at the time of their formation.