A landing page is a standalone web page that is designed with a single message, found when a visitor clicks on a Pay Per Click (PPC) ad or a search engine result. They are used in marketing campaigns to convert visitors into customers by providing a clear message and a call to action. Using a URL (mydomain.com/ppc) specific to a PPC or advertising campaign makes it easier to see how well your landing pages are doing.
You want to direct your paid traffic to a landing page instead of your homepage, in most cases, because the home page may be referencing multiple products or services making it more difficult to get your intended message across. The purpose of a landing page is to provide a targeted message to your potential customer.