There are two distinct types of hardened lava you’ll likely see in Hawaii – pahoehoe and a’a.  These are Hawaiian names that have been adopted universally for describing the two main types of lava.

Pahoehoe (pronounced like “pah hoy hoy”) is a smooth lava produced when lava flows slowly and gently.  A’a (pronounced like “ah ah”) is a very rough and rugged lava that is produced when lave flows quickly.  Here are some photos that will help you distinguish the two types of lava.

This photo from my personal collection shows cooled pahoehoe:

The photo below shows a’a lava in action.  Notice the rough texture on top. It is believed that the name a’a came from the noises someone might make while walking over it in bare feet.  Maybe an alternative name should be ouch’ouch or ow’ow. (Photo use per Wikimedia Commons)

For more on lava, see this Wikipedia page.