A Cost Per Lead (CPL) is when there is only a cost when there is a lead.
A Lead is when someone is interested in doing your desired action. This can normally be showed by them completing another action on your site.
For example, Leads can include a phone call or an email. A lead is any action where they have engaged with the company and shown an interest in the product/service provided.
A CPL is a performance indicator that is put into business reports to track how well the site is doing.