A'a is a type of lava flow in which the lava forms clinkers (broken bits of solidified lava) on its surface, forming a very rough and irregular rock layer. The resulting lava has a very jagged surface, with sharp edges ('spinose'), and is painful to walk over.
A smoothly flowing lava flow is called pahoehoe. The surface of pahoehoe can stretch, wrinkle, and tear to adjust to the movements of the lava flowing beneath. Once pahoehoe cools and becomes more viscous the surface hardens and becomes inflexible. If the lava flow advances over steep or rocky terrain, the crust will crack and break, pulling apart in jagged clinkers or smoother blocks which are drawn along on the surface of the flow.
As the flow advances, the clinkers fall off the top of the flow and down the front, and are plowed underneath. When the flow cools, this will result in a formation that looks like a sandwich, with a layer of clinkers above and beneath, and a solid, dense core in between. The clinker layers are often meters thick. Generally speaking, the thicker the clinker layer, the less spiny the clinkers are.
Very viscous lava flows may form large blocks rather than clinkers; this is called block lava or blocky lava, and is generally considered to be a different type of flow than a'a. Block lava has a thicker crust, and less churning than a'a, resulting in rough blocks rather than hashed rubble.
While pahoehoe tends to form all kinds of really cool formations, a'a doesn't. A'a may form giant accretionary balls, up to several meters in diameter, in which a ball of lava is carried along on top of a flow, picking up globs of lava as it rolls and growing bigger and bigger. But a'a's primary claim to coolness is it's appearance when it is actually flowing. A'a tends to advance slowly, with a glowing red interior beneath a blackening crust of irregular torn jags. It makes a clinking sound as it advances, as the clinkers fall off the front of the flow and are crushed.
A few boring notes on terminology:
A'a flows are subdivided into two types; proximal a'a flows and distal a'a flows. 'Proximal' and 'distal' refer to the distance from the volcano vent, but while there is a correlation, distance from the vent is not what determines the nomenclature. Proximal a'a flows are hot and fast, up to 3 meters thick, with very spiny clinkers, and the central rock tends to be less dense and more vesicular. These flows would be pahoehoe but for the exceptionally rough terrain that they are flowing over. Distal flows are slower, much thicker, denser, and more likely to form blocks. These two types of a'a flows are the endpoints to a continuum, with many gradients in between. A'a is formed from lava with a high gas content, while pahoehoe forms from lava with a low gas content.
A'a is a Hawaiian word, used both for this type of lava and also to mean "burn" or "blaze". It is pronounced as ah-ah (but a 'sharp' ah; not 'ahh'. It is a back-of-the-mouth ah). The correct spelling would be ʻaʻā, although almost no one bothers to hunt down all those weird symbols. It's common to see both a'a and simply aa in the writings of volcanologists.