Lead Generation business models are those where a company uses the web, or some other mechanism, to generate a large number of leads, and sells them on at a higher price.
A well known example of this would be LendingTree.com, where they use radio advertisements, Google Adwords, SEO, etc. to drive leads to their site, and then sell these leads to banks and mortgage companies.
Another good example of this is a company called QuinnStreet. QuinnStreet works in a series of verticals to develop proprietary content sites, publisher partnerships, ad placements, email campaigns, etc. to develop a lead flow, and then sells that on to buyers at a higher price per lead than they are paying overall to acquire the leads.
Going back to our CAC and LTV thought process, these companies clearly have a higher LTV (customer lifetime value) than their CAC (cost to acquire that customer). These can be very attractive business models.