The Yamas - A yama (Sanskrit) यम, literally "death", is a rule or code of conduct for living which will help bring a compassionate death to the ego or "the lower self". The yamas comprise the "shall-not" in our dealings with the external world as the Niyamas comprise the "shall-do" in our dealings with the inner world.
1) Ahimsa - Non-violence - cause no harm to any living creature
This is why a lot of yogi's are vegetarians, but this also means not gossiping about people, not being rude to people, every action is supposed to come from a place of not injuring another either through actions, words, thoughts.
2) Satya - right communication, truthfulness, refrain from telling lies, refrain from deception
No lies at all, even to ourselves, our intention in every situation is to be truthful
3) Asteya - non-covetous - non-stealing, resisting desire from that which does not belong to us - do not take that which is not freely given
This means no stealing, even stealing one's thoughts, no burning discs from others, nada
4) Bramacharya - celibacy, moderation in our actions, moderation in our thoughts
While many focus on the celibacy piece of this, one does not have to be celibate but moderate in thoughts in actions which means, thoughtful, not extravagant, no gluttony
5) Aparigraha - non-greediness, determine what is a true need vs a desire
This can stretch even to not coveting another's yoga pose!