A 'Lead' in our reports and goals are visits to the client's web pages that contain our 'Lead' tracking pixel, which we provide in Account Preferences.
You can place it on Thank You confirmation pages wherever you determine a 'Lead' conversion takes place, it's different for every company.
Some consider an eNewsletter signup a Lead, they would place the pixel there. Other's don't consider that a Lead, so they wouldn't want to place the Lead pixel there.
You can actually place multiple pixels on one page.
In the above example, you might want to create a custom tracking event for eNewsletters and place the custom pixel AND the Lead pixel, so this, and other Lead conversion events are counted towards the company's total Lead count in reports.