Opportunities are potential revenue-generating events, or sale to an account, that needs to be tracked through a sales process to completion. The different sales stages the opportunity moves through for example, initial contact, analysis, proposal, negotiation etc indicating the value of your sales pipeline by sales stage.
LEAD = lead is a contact or company in your system that has in some way indicated interest in your products or services, but has not yet been qualified by your marketing or sales teams.
OPPORTUNITY = Once there is a budget and a timeline, there must be both, a lead becomes an opportunity. It signals the beginning of your sales process with a potential or existing client.