Basalto is considered a mafic silicate rock. Among other characteristics, mafic minerals and rocks are generally dark in color and high in specific gravity. This is in large part due to the amount of iron, magnesium, and several other relatively heavy elements which "contaminate" the silica and oxygen.
Basalto is extrusive. The magma from which it cools breaks through the crust of the earth and erupts on the surface. We call these types of events volcanic eruptions, and there are several main types. The volcanoes that make basalto are very common, and tend to form long and persistent zones of rifting in nearly all of the ocean basins. We now believe that these undersea volcanic areas represent huge spreading ridges where the earth's crust is separating.
Because the magma comes out of the earth (and often into water) it cools very quickly, and the minerals have very little opportunity to grow. Basalto is commonly very fine grained, and it is nearly impossible to see individual minerals without magnification.
Basalto is most commonly used in construction in form of paving stones, interior design as floor tiles and stepping stones, but also for making cobblestones.