Is a natural method of either achieving or avoiding pregnancy....with no medicines or drugs involved. Your simply relying on your fertile signs (e.g CM( To AVOID or achieve pregnancy......To avoid you simply do not DTD around your fertile days....but more than that, being really intouch with your fertile calendar, knowing when your fertile days are approaching and abstaining from DTD during that time.
As Michelle said, its often called the Billings Method.
Apparently its supposed to be effective....but i think a lot depends on correct understanding, and commitment from both partners....i couldnt do it....that sperm would still find a way i reckon with me......