Lead: is a potential Customer, someone who can give you business.
Customer: is an individual or organization who has given you business.
Contact: is a person who belongs to the Customer. This is applicable in business-to-business scenario, where you may contact multiple individuals belonging to the same customer.
A Lead can be converted to a Customer by selecting “Customer” from the Make
dropdown. Once the Customer is created, the Lead becomes “Converted” and any
further Opportunities from the same source can be created against this
Next: Sales Funnel