First, it might be helpful to understand what INDEX means when it comes to search engines. All of the search engines have robots that crawl the internet, identifying web pages and determining what each page is about. Once a page has been crawled, it is either added to the index or not. The index is the massive database of content that the search engines pull from to offer up responses to users’ search queries.
The goal of most web pages is to be “visible” and rank highly in the search results. In other words, they want to be included in the index, and they want to convince the search engines of their relevancy so they perform well in search results.
NOINDEX is an instruction that you put in your website’s code, in the robots meta tag, literally telling the search engines NOT to index that particular page or website.
Why would you want Google to leave certain pages out of its index?
If you’re building a page for SEM purposes, and you’re using an exact match domain rather than hosting the landing page on your main website, providing the NOINDEX directive will keep Google from penalizing your site for spam. Otherwise, the search engines see those EMDs as an attempt to game the system and dominate the organic results, and will not only drop them from the index, but the negative effects could spill over onto your main website if the sites are linked.
If you host your PPC landing pages on your main domain rather than using EMDs, it’s still a good idea to use NOINDEX since these pages tend to be very similar in content and may be keyword heavy, which looks like low-quality spam to the search engines.
Designating these pages as NOINDEX keeps them out of the organic search results, but they are still perfectly functional as PPC landing pages.
There are a variety of ways to implement a NOINDEX directive. Here are the specifics from Google.