Brief explanation:

Pahoehoe lava is a common type of basaltic lava that cools down forming typical smooth, billowy, or ropy surfaces. Often, it forms by many small breakouts of lava from an over-crusted inflating flat, irregular flow. It is typical for shield volcanoes such as on Hawaii, where the name (meaning something like "good to walk") comes from.
Pahoehoe lava forms from hot basaltic lava when the viscosity of the lava (e.g. because of low gas bubbles content and high temperatures) and/or the strain rate of the flow (related mainly to eruption rate and steepness of the ground) are low. When these factors change, the same original lava can produce the other end-member known as a'a-lava, which has a fragmented, rough, sometimes spiny, or blocky surface. In addition, transitional types of lava between both a'a and pahoehoe lava can be found.