Pahoehoe refers to a type of lava flow; these are smooth-topped lava flows, caused by hot fluid lava flowing easily and smoothly. When the surface of the lava cools it forms a crust, but underneath the crust the red hot lava keeps flowing. As the lava flow expands the crust folds, wrinkles, cracks, and rips, allowing more lava to spill thru; this lava crusts over in turn. Depending on the heat, speed, and composition of the lava, pahoehoe can cool into all kinds of odd forms; pillows and ropy folds, toes, blisters and coils. Pahoehoe flows are sometimes referred to as lava sculpture.
Although pahoehoe is referred to as 'smooth lava', it is actually a rough-grained, basaltic rock. There is an outer crust about 1 cm thick that is very glassy; next is a transition zone of a few centimeters, before finding the core of the rock, which is also called pahoehoe. The crust may be silvery, a glassy dark-blue, or black.
Pahoehoe is one of the two main types of lava flows. The other is aa (or a'a). Aa is formed when the lava picks up broken bits of lava, rubble, and clinkers, rolling into broken clumps and balls, and generally making a mess of things. Pahoehoe flows may turn into a'a flows as they cool and reach virgin landscape. Pahoehoe flows often cover cooling a'a flows, as the hot a'a is warm enough to keep the new lava from cooling and breaking.
As the lava flow drains out from under the hard pahoehoe crust, lava tubes may form, forming long tunnels, some big enough for a human to walk through. Shelly pahoehoe refers to lumps of rolls of pahoehoe that have been drained from the inside, to form a complex of smaller fragile 'lava tubes'; these can be dangerous to walk over, as the shells may break, causing you to fall or the sharp fragments of the crust to cut your lower legs.
Some other forms of pahoehoe:
Ropy pahoehoe: A very common type of pahoehoe. This forms when a partially cooled crust is bunched up and wrinkled, due to the turbulence of the underlying lava flow. The resulting folds look something like coiled ropes, but perhaps even more like some fabric bunched and wrinkled into fine folds. Ropy pahoehoe may appear in the form of lava coils, where the lava piled up gently into a spiral, looking like an oversized Ammonite.
: Another very common formation. A toe is a budding out of the lava flow, a smaller protrusion from the main mass of lava. Lava toes may have ropy or smooth (or other) surfaces, but usually they are smaller, smooth flows occurring at the edges of the flow. As toes fill and grow with new lava, they may bud new toes of their own.
Pahoehoe entrails: Pahoehoe that was formed by fast moving lava flowing down a steep slope. Long flat tongues of lava that look something like an animal's entrails spilling over and around each other in a giant tangle. Not having butchered many animals myself, I would have compared it to melted wax, but to each their own.
Pahoehoe blister: Small glass bubbles on the surface of pahoehoe formed when gasses try to escape the flow, but are captured in the forming glassy crust. Somewhat akin to Limu o Pele, but much stronger and thicker due to the less explosive nature of the gasses and slower cooling time.
Spiny pahoehoe: Pahoehoe without the glassy crust. Formed when viscous, cooler lava cools at a rate slow enough that there is relatively complete crystallization of the rock, and glass does not form. The surface is rough, or spiny. It is also called toothpaste pahoehoe or sharkskin pahoehoe.
Rubbly pahoehoe: Usually we think of lava as either 'smooth' pahoehoe and 'crumbly' a'a. This works pretty well with the Hawaiian volcanoes, but in other parts of the world it isn't uncommon to see a cross between the two. Rubbly pahoehoe is a pahoehoe flow in which the flow top is composed of broken bits of small pahoehoe lobes (whereas in aa the top of the flow is composed of clinkers). It's debatable whether this should actually be considered a type of pahoehoe flow, but it is a lava formation that has to do with pahoehoe.
Pahoehoe is a Hawaiian word, now used regularly in English as the proper name for this sort of lava. In Hawaiian it should be written pāhoehoe. It may be pronounced either 'pahoyhoy', 'paHO-ehHO-eh', or 'puh-hoh-ee-hoh-ee'.