To howl is to make a long, sad, crying sound. You might howl every once in a while, but it's more common to hear wolves and dogs let loose and howl, especially in the presence of a full moon.
Terribly sad or fearful people howl, and wolves howl to communicate with each other. Your dog might howl at the sound of a passing fire truck, which also makes a noise with its siren that you can call a howl. Before the 1400s, the word was houlen, a word that probably emerged as an imitation of the way a howl sounds.