The four words are often confused because they are similar in meaning. They are all used when something has an effect on something else. However, there are some subtle differences in meaning between them.  "Affect" is a verb. It means to produce a change in something.  "Impact" is usually a noun. It refers to the influence or effect that something has.  "Influence" is a verb. It means to have an effect on someone or something, especially in a way that is not immediately obvious.  "Effect" is usually a noun. It means the result of an action or situation.  Some examples might help to illustrate the differences between these words: 1. The abolition of child labor had a positive effect on society by giving children the opportunity to go to school instead of work.  2. The recession had a negative affect on my business because many people stopped buying luxury items such as cars and jewellery.  3. My boss's retirement will have an impact on the company because he was such an important figurehead.  4. I try not to let other people's opinions influence my own too much.