How many other sites have links to your site? Incoming links can be crucial to your site's rankings with the search engines. 

If lots of other people are linking to your site, the search engines figure that you must be important. It is a popularity contest of sorts.

How do you know if you have incoming links and where they are coming from?

If you have your site associated with Google Analytics or Google Webmaster tools, you can view which sites have links to your site. You can also see how many people are reaching your site from those links.

Google Analytics

Acquisition - Overview

Go to the control panel of your Google Analytics  report pages, scroll down the left column to Acquisition -> Overview.

The Acquisition section shows you how people are getting to your site.

You will see a pie chart labeled: Top Channels and a table below that contains specific numbers. This pie chart is displaying the ratio of the following methods that people have used to get to your site:

Organic Search: This means that the visitor typed a search phrase into the box of a search engine. This could be Google, Bing, Yahoo or any other search engine.

Direct: This means that someone actually typed the URL for your web page into the address bar or had a shortcut created in their browser.

Referral: This is the incoming link section. An incoming link means that some external site, with a domain name other than yours, had a link to your site. A visitor clicked on this link to reach your site.

Social: This means that someone got to your site through a post on a Social Network site such as Facebook, Twitter, LinkedIn, etc.

Acquisition - All Traffic

Also in the list is a section titled All Traffic. This contains a table with a breakdown of each source that people used to get to your site.

If you want to see exactly which sites have links that point to your site, you can look at this list.

Why are incoming links important?

Besides aiding your rankings with the search engines, incoming links mean more traffic to your site. You can sell more products or get my impressions for your advertisements. 

Getting higher in the search engine lists is getting more and more difficult as hundreds of new sites are launched every day. You need to consider new ways to get visitors to your site other than having them stumble across your site with a search engine query.