A lead is a potential customer who might be interested in your products or services.
Opportunity is a qualified lead i.e. when they decide to consider your offering.
A customer, who is someone who receives services or products from a seller for a monetary consideration.
Contact represents a person in ERPNext, and can be associated to a customer or a supplier or a company
You can record the addresses associated with a Lead, Customer, Supplier, Shareholder, Sales Partner or a Warehouse.
A Campaign is a full-scale implementation of a sales strategy to promote a product or service.
A Lead Source is a source from which the leads are generated.
Opportunity Type indicates the broad category of opportunity like Sales, Support, Maintenance, Partnership etc.
A Sales Stage indicates the level at which an opportunity is in the sales cycle.
Sales Person is a person who sells your products/services.
Customer Group is an aggregation of customers that are similar in some way.
You can analyze your sales funnel and quantify the number of prospects at each stage of the process and you can get an idea of your potential customers.