A lead is information that you get about a person or business that could turn out to be a sale. The information for leads is usually the name of the person or business, an email address and maybe some other information about the person or business like their mailing address, their phone number or any other important information you get about that person or business.
A sale when someone buys something from a person or business, plain and simple. More sales is more money for you or your business.
Cheers, Dean.