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
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