A livelihood is the work we do in order to live. To be ‘right’ in the full sense of the word, a livelihood would; (1) have to provide one with at least one’s basic needs, (2) it would have to be in accordance with ethical principles and (3) it would have to make a useful and hopefully a beneficial contribution to society.
The Buddha mentioned as examples of wrong livelihood, dealing in weapons, human beings (slavery, people smuggling, certain types of prostitution and living off the income generated by it), trade in flesh, manufacturing and selling alcohol and poisons (A.III,207). Today this list could be increased considerably. The Buddha described a wholesome and honest income as being ‘earned by hard work, by strength of arm and sweat of brow, honestly and lawfully’ (A.II,67). For example, whether living off investments would be a right or wrong livelihood would depend on what one’s money was invested in. We would normally think of being a doctor, a nurse or a teacher as being right livelihoods but again, it would depend just how these professions were practiced. So right livelihood is not just what type of work we do, but also how we do our work.
Abstention from making a living that harms others, including trading weapons, trading in human beings (such as slavery), intoxicating drinks, narcotics, poisons, handling animal flesh such as a butcher, or killing animals.
The Buddha taught and lived the philosophy of ahimsa, or nonviolence. The Buddha had great compassion for all beings, not just humans. The Buddha understood that all beings feel pain and can suffer. For these reasons, the Buddha was opposed to specific professions which are aimed at harming people, animals, and / or the environment.
In the Buddha’s teachings, what matters most is the intention. Occupations which are specifically designed to harm another are not recommended. Some occupations which may appear to be harmful, may not be when you consider the intention. An example, is a casino black jack dealer. Gambling is certainly an activity that can end up doing much harm, with lost wages and wealth for a family. But, this is only if it gets to an addictive level. In moderation it can be a form of entertainment, just like watching television. A casino black jack dealer has no intention of creating a gambling addict anymore than a TV cable installer wants to create a TV watching addict. Therefore, a casino worker is not a wrong livelihood.