Odoo provides two primary documents to manage interactions with your customers or potential customers. You can think of leads as less critical—and perhaps less likely to turn into a real sales situation—than an opportunity. A good example of leads would be the few dozen business cards you get from people you met at a conference. You could add each of them as a lead for further follow-up. An example of an opportunity would be meeting someone at the conference and having a detailed conversation on how your company provides appropriate services.
Many people get confused between when to use leads and when to use opportunities. The best way to remember the difference is that leads are intangible and are essentially potential contacts. Opportunities should be more clearly defined, have some sort of expected income if successful, and provide significant project details and scope compared to a simple lead.