Brief explanation:

Lava flows are almost self-explanatory. When magma is erupted in molten or a partially molten state it often has the ability to flow. This is typically the case for basaltic volcanoes such as Hawaii and Etna whose lavas are relatively fluid. Lava flows might form either as primary flows directly flowing out of a vent from the vent or by rapid aggregation of hot fluid spatter that fall back from lava fountains to form a flow.

The appearance of lava flows can vary greatly with chemistry, flow rate, strain rate, temperature, viscosity and other factors. The most simple division can be made between aa lava that usually has a blocky appearance, since the surface of the flow breaks into spiny or blocky pieces, and pahoehoe lava flows, which form a smooth surface and often produce spectacular ropy textures.

More on lava flows and their various types can be found on a brilliant section of Vick Camps "How volcanoes work" website.

More lava flows from Etna in 2001, Stromboli in Jan. 2003 and Hawaii in 2000/2001:

Above in middle row: typical example of very fluid pahoehoe lava.