Referrer: In the world of Search Engine Optimization (SEO), referrers are the sources from which web traffic comes from. For example, a referrer could be an email, a social media post, a link from a blog, another website, or even a search engine.

It's important to keep on eye on referrer data in SEO, because as they say, if you can measure it, you can improve it. Keeping track of how much web traffic is coming from which sources can help webmasters create better SEO strategies.

Though there are several different types of referrers, the three most important ones that websites need to keep track of are direct navigation, referral traffic, and search traffic.

Direct navigation refers to any traffic that comes from typed in addresses (visitors who type a site's URL in and go directly to the site), bookmarks, email links without tracking codes (like personal emails), and so on. It's the type of visitor who goes straight to a website before visiting any other place on the web.

Referral traffic is the type of traffic that comes from links, whether they're links from around the web, from trackable emails (like ones sent out in a marketing campaign), social media, and so on.

Search traffic is any traffic that comes through as the result of a query on any major or minor search engine.

Measuring and analyzing such data can help a website figure out which referrers it's best taking advantage of, and which ones they can use to improve. Keeping a vigilant eye on referrer data can help webmaster compare trends over time, allowing them to figure out ways to bring in more traffic.