What is SEO? Search Engine Optimization Explained

SEO (Search Engine Optimization) increases a website’s quantity and quality of organic search traffic.

Free, passive website traffic month after month.

How do you optimise content for SEO? What “ranking factors” matter?

To answer, we must understand search engines.

How search engines work

Search engines are digital libraries.

They store web pages, not books.

When you enter a query, a search engine searches its index for relevant results

The algorithm does this.

Nobody knows how these algorithms work, but Google has hints.

“How search works” says:

Search algorithms consider your query, page relevance and usability, source expertise, location, and settings to give you the most relevant results. The weight of each factor depends on the nature of your query; for example, freshness is more important when answering questions about current events than dictionary definitions.

Most people use Google for web searches. It has the most trustworthy algorithm.

You can also optimise for other search engines.

Our search engine guide explains more.

How SEO works

SEO shows search engines that your content is the best for a given topic.

All search engines want to show users the most relevant results.\

How you optimise depends on the search engine.

If you want more organic web traffic, you must understand Google’s algorithm. YouTube’s algorithm determines video views.

It’s impossible to cover every search engine’s ranking algorithm in this guide.

We’ll focus on Google going forward.

How to optimize for Google

Google’s ranking factors number over 200.

In 2010, some predicted 10,000.

Some ranking factors are known, but not all.

How? Because Google told us, and many people—including us—have studied Google rankings.

We’ll discuss some soon. Google ranks web pages, not sites.

Every page on your site shouldn’t rank for “stained glass windows” just because your business makes them.

Different pages can rank for various keywords and topics.

Let’s talk about ranking and search engine visibility.


Google must know about your content before it can rank it.

Google crawls the web to find new content, but it’s primarily used. Google crawls by following links on known pages to new ones.

They use a spider to do this.

Your homepage has a backlink from a Google-indexed website.

Next time they crawl that site, they’ll follow the link to your homepage and likely index it.

From your homepage, they’ll find other pages.

Some things can block Google crawlers:

Google crawls your site via internal links. Pages without internal links aren’t crawled.

Google doesn’t crawl nofollowed links.

Exclude pages from Google’s index with a noindex meta tag or HTTP header. If your site’s internal links come from noindexed pages, Google may not find them.

robots.txt: Robots.txt tells Google where to go on your site. This won’t crawl blocked pages.

If any of these issues concern you, run an SEO audit with Ahrefs Site Audit.


Google searches from mobile devices are 63% and growing.

In 2016, Google announced a ranking boost for mobile-friendly websites in mobile search results.

Google switched to mobile-first indexing in 2018, using the mobile version of your page for indexing and ranking.

Here’s a crucial Adobe statistic:

8 in 10 consumers would stop engaging with poor mobile content.

When a mobile site’s desktop version loads, most people will hit back.

Google wants satisfied users. Mobile-unfriendly pages cause dissatisfaction. Even if you rank and get the click, few will read your content.

Google’s mobile-friendly testing tool can check your web pages.

Hire a developer if not.


Your page’s speed. Desktop and mobile rankings use it.

Why? Again, Google wants happy users. Unhappy users click on slow search results.

Google’s Pagespeed Insights tool checks website speed.

Search intent

Finding keywords is easy. Paste a topic into Ahrefs Keywords Explorer to find relevant keyword ideas.

Many people overlook whether their page matches their keyword’s search intent.

Example search intent

Google results for “slow cooker recipes”:

Compared to “slow cooker”:

Google shows two different search results despite similar keywords. For “slow cooker recipes,” they list many. They show product listings and category pages for “slow cooker.”

Google interprets the query and shows relevant results.

Intent search.

How to optimise?

Examine top-ranking pages to determine the “3 C’s of search intent.”

Most results are blog posts, product pages, category pages, or landing pages?

How-to guides, list-style articles, tutorials, comparisons, opinion pieces, or something else? (Note. This mainly applies to informative topics.

Perspective: Is there a theme or USP across top-ranking pages? If so, you’ll know what searchers value.

You can also check SERP features for intent.

A featured snippet in the results may indicate that the searcher wants information.

Ahrefs Keywords Explorer lets you filter keywords by SERP features.


PageRank is Google’s ranking algorithm.

Backlinks are essentially votes. Voted-upon pages rank higher.

Why? We studied nearly 1 billion web pages last year and found a correlation between referring domains and organic search traffic.

Backlinks are important for ranking for anything worthwhile.

Links to product pages can be difficult to build.

If you’re new to link building, focus on your best informational content (e.g., a blog post or a free tool).

Here’s how:

Google the keyword. Look for inferior pages. Paste the URL to see its top 100 links.

Explain why your content is better and ask if they’ll swap links.

Skyscraper Technique describes this strategy.

These articles and videos explain this and other backlink building techniques.

Explain why your content is better and ask if they’ll swap links.

Skyscraper Technique describes this strategy.

These articles and videos explain this and other backlink building techniques.



not duplicate Some are heavier.

PageRank incorporates this fact.

High-authority backlinks are stronger than low-authority ones.

Google stopped publishing PageRank in 2016. There’s no way to tell a page’s “authority” in Google’s eyes.

Ahrefs URL Rating is a similar metric (UR).

URL Rating is a 0-100 scale that considers a page’s backlinks’ quantity and quality.

We found a positive correlation between UR and organic search traffic.

When building backlinks, prioritise links from strong pages over weak ones.

Ahrefs Site Explorer’s UR column is the best place to look for backlink opportunities on competing pages.

Backlinks aren’t the only way to increase a website’s “authority.”

URL Rating (UR) also considers internal links, which boost a page’s authority.

If you’re having trouble building backlinks to a page, consider adding internal links from high-authority pages.

Ahrefs Site Explorer’s “Best by Links” report shows your most authoritative pages.

Don’t force links where they don’t belong. Contextualize links.

This tactic boosts the “authority” of product pages. It’s difficult to directly link to those pages.


Content quality

Google wants reliable and useful results always.

They look for expertise, authority, and trustworthiness in content.

This is EAT.

Google’s Search Quality Rater Guidelines explain EAT.

Other ways to improve content quality:

7th or 8th grade reading level. This level is typical for Americans.

Be concise. This isn’t an essay.

Link to relevant resources. Don’t “hoard PageRank.” Create valuable content for visitors.

Avoid text walls. Add images, quotes, etc. Be skimmable.

Accessible content is better for most searchers.

Some searches value freshness.

If you Google “best router,” almost all of the results are new.

Technology advances quickly. Nobody cares about 2016’s top routers. It’s useless.

Freshness matters less for other queries.

Top result for “tie a tie”:

The page hasn’t been updated in over six years, but tying a tie is the same now as it was then.

See if freshness is a ranking factor for your target keyword. Strategize as needed.

Why ranking is overrated… kind of

Google uses location, search history, and search settings to “tailor your results”

Even if your site ranks #1 for your target keyword, it may not be for everyone.

Searching “flapjack recipe” in the UK vs. the US yields different results.

Why? British flapjacks are oat bars. American pancakes.

Use an incognito tab to check “true” rankings and remove search history personalization. VPN can help with location.

You can also use Ahrefs Rank Tracker to track keywords by zip code. This helps local SEO.

Rankings fluctuate, though.

Here are our 2018 “SEO audit” rankings:

Because of this, organic traffic is often preferable to rankings.

Google Analytics or Ahrefs Site Explorer can help with this.

Paste a URL into the “Overview” report’s “Organic traffic” tab.

You want a graph like this for your site’s important pages:

Many pages rank for thousands of keywords, so it’s better to focus on traffic. Many of these keywords drive traffic, not just one.

Final thoughts

Knowing how search engines work and what they look for when ranking content is crucial for ranking content.

Search engine algorithms change constantly, so what’s important today may not be next year.

Don’t worry. The important things remain constant.

Backlinks, “authority,” and matching search intent have been important for years, and that won’t change soon.

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