Lead is an organisation or a person who can be a potential buyer of products and services offered by the selling organisation. For example, you meet a person from a organization in an event and exchange business cards. The person might not have shown direct interest in the product and services of the selling company, but you identify that his/her company uses similar kind of products. Then this person is added to the system as a lead, since you expect that you may be able to break the competition and close a deal in the future.