Since you asked

Betel Nut is the fruit of an Areca Palm which gives a mild narcotic effect when chewed.

These green pods can be chewed as is but most who chew cut them open and add a limestone (or coral) powder and tobacco. Then they wrap the whole thing in a pepper leaf. The packet is then stuck between the gum and the cheek much like chewing tobacco.

Chewing changes the saliva red and ends up staining the teeth of those who chew regularly. It is also known to produce mouth, throat, and stomach cancers in those who chew.

The difficult thing is that on the Island of Yap betel nut chewing is so prevalent, it has become part of how they communicate. While on Yap over break, it was very interesting to watch since almost everyone chews constantly. Just one after the other and it is culturally accepted and encouraged. Then they come to school and are not supposed to chew at all but many of them are addicted to either the nut or the tobacco. Both are prohibited on campus.

Apparently from what I read in the web, Betel nut chewing started in south east Asia but it is now wide spread. There are some interesting videos on YouTube about Betel Nut chewing if you are still curious and have some free time on your hands.