Pretty much means that the person usually doesn't actually have to buy something (provide a credit card).
A lead is when actually buying and/or credit card information is not needed for you to receive credit. It's usually when your visitor ONLY has to fill out a form or email. It's a lot easier to get leads then actual sales..
Sale: customer buys something Lead: customer does not buy anything, but signs up for a newsletter, downloads a free trial of software, adds name to an email list etc.....