A sales lead is the Identification data of a person or entity that has the interest, authority and budget (i.e. money) to purchase a product or service.
This step represents the first stage of a sales process.
Sales lead definintions vary from company to company and the term is sometimes used interchangibly with "sales prospects".
That said, the difference is pretty trivial: depending on your internal definition, your pipeline can begin with leads (less qualified) which then turn into prospects (more qualified) - or the other way around.