Bestiality refers to humans engaging in sexual behaviors and activities with non-human animals. There are references to humans having sex with non-human animals throughout history, and since people began filming humans having sex, there has been a genre of pornography featuring humans having sex with animals.
In many countries it is illegal for a human to have sex with an animal, although the ways that these activities are regulated differ. Bestiality may be distinguished from another term, zoophilia, which refers to a human's experience of having loving or erotic/sexual feelings for certain non-human animals. Whereas the term zoophilia refers to a feeling or attachment, bestiality only refers to the activity.
Because of social stigma and legality, it is hard to know how commonly bestiality is practiced. Historically people have thought that it was more commonly practiced in rural areas, although more recently researchers and others have suggested this may not be true.
Miletski, H. Understanding Bestiality & Zoophilia Bethesda: East-West Publishing, 2002.