Brief explanation:

Basalt is the usually hard and black volcanic rock formed from (liquid) balsalitc lava. Balsaltic lava contains less than about 52 percent silica (SiO2) by weight. Because of its low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can quickly and easily flow more than 20 km from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns, although basaltic lava fountains and fissure eruptions, however, still can be hundreds of meters tall. Basaltic lava is erupted at temperatures between 1100 to 1250°C.
Basalt is by far the most common volcanic rock type. Basaltic magma is formed by partial melting of material from the upper mantle, and and is therefore typical for volcanism at hot-spots and at rift-zones. In these areas, upwelling of the mantle (either caused by a rising mantle plume underneath hot-spots, or by a divergent plate boundary at mid-ocean rift zones) decreases the pressure of the hot rock and therefore causes (partial) melting.
Oceanic crust and submarine volcanoes consist largely of basalt, because most of them are formed at rift-zones (all ocean floor) or hot-spots. Among subaerial volcanoes, basaltic lava is primarily found at shield volcanoes.

Basaltic lava flows can be subdivided into two end-member structural types, according to their flow surfaces:

-- Pahoehoe lava - smooth, billowy, or ropy surface.
-- A'a' lava - fragmented, rough, sometimes spiny, or blocky surface

More about volcanic rocks (chemical and physical properties) can be found at a special page of the USGS.