Let's answer that by first establishing what a landing page is not. That's important because there are a lot of different definitions floating around out there. (The other definitions aren't better or worse -- they're just different.) The biggest difference I often see is the term "landing page" being used to refer to any ol' page on a website. You know, they're all pages on which one might "land."
However many marketers, ourselves included, prefer to differentiate landing pages a little bit more than that. Instead, we would call any page on the web on which one might land a "web page," whereas a "landing page" is any page on the web on which one might land that 1) has a form and 2) exists solely to capture a visitor's information through that form.
In other words, all landing pages are web pages, but not all web pages are landing pages.