Click-through landing pages are designed to facilitate a single objective - to click on the primary call to action (CTA) to progress to the next step in your sales funnel.
Typically used for product based ecommerce, click-through landing pages enhance the upstream ad message (paid search, email or banner ad) and explain the features and benefits of the item being offered.
Important characteristics of a successful click-through landing page are: focus, message clarity, on-brand design integration and a targeted value proposition.
Jen Gordon has produced some great landing page designs for her iPad app development company ACleverTwist. The first example here is a click-through e-commerce template with the goal of buying an iPad app from the iTunes Store.