This lead capture software ranges from simple to complex. You can have a landing page that captures the data, or you can sign up for a system that tracks your user's accounts once they sign in. You can see what products they looked at, what pages they read and, judging from their account activity, you can see how likely they are to become customers.
If you're wondering what a sales funnel is, simply imagine a real-world funnel. At the top of that funnel, some substance is poured in, which filters down towards one finite destination. In sales, something similar occurs. At the top, lots of visitors arrive who may enter your funnel. However, unlike the real-world funnel, not all who enter the sales funnel will reemerge out from the other end.