First off a Landing Page is typically a single web page that is tied in with some other marketing effort – Direct Mail, Web Site Call-To-Action, Pay-Per-Click Online Advertising, Print Advertising etc.
So what is a Landing Page used for? Well a Landing page is use as a lead generation tool. The main goal of a landing page is to give your prospect enough information to take action and sign up for whatever offer you are making. A Landing Page will use a form to collect data on your prospect. Click on this link to see an example of a Landing Page design.
This is something you need for your web site. The goal of the web site is to get a lot of traffic, then how do you convert this traffic into customers? One way is Landing Pages. By making and offer to your visitors, you are able to have them take the next step and move one step closer to becoming a client. There is an art and a science to Landing Pages though so you need to do several tests to see what works for your product or service.
The key is to have some sort of analytics to be able to track what kind of activity you are getting to your landing page beyond just the people that fill out the form page. Take this data and compare it to the next page you create and don't forget to test something different– layout, offer, headline etc.