Ratha,
We may have different definitions of a landing page. The reason I say that is most landing pages that I am familiar with are the pages that a users lands on when they hit your site, the first thing they see.
For the sites that I have developed for most of my customers that is the index.php or index.html page of their website. That is where the term landing came from. It is the page they land on for their visit.
Now, with that said. I have had customers request a special landing page for their products area of their website. The first page a customer visits when they reach a certain section of the site, normally a products area. I normally do not encourage that as a lot of special attention goes into developing a good landing page as far as color choice, content, eye appeal, ads, placement, etc. There is a good possibility that many users will never hit the page if they have to navigate to it. So I tend to encourage my customers to make that the home page of the site the landing page. It is about a 70% / 30% mix as to those that use the home page and those who do not.
Landing pages are designed with a single objective. To increase sales with a CTA (Call To Action) focus. Drives people to purchase what you are trying to sell. Normally driven by a PPC (Pay Per Click) campaign, they are geared to sell products.
A well designed landing page can bring in a tremendous amount of ROI (Return On Investment) if it is done well. They can be tricky to get just right, and with the changes in the consumer market and what does and does not appeal to consumers in a constant state of flux, they need to updated and tweaked often. It is kind of like hitting a moving target, the parameters are constantly changing.
I hope this helps.