Odoo provides two primary documents for managing 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 that you get a few dozen business cards 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 if you met someone at a conference and had 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 is that Leads are intangible and are essentially potential contacts. Opportunities should be more clearly defined, have some sort of expected income of successful, and provide significant project details...