On Friday (November 4th 2016) the official google blog announced that they are now going to be using the mobile version of the site BEFORE using the desktop version of your site for evaluating it’s placement within their search engine.
our algorithms will eventually primarily use the mobile version of a site’s content to rank pages from that site (source: https://webmasters.googleblog.com/2016/11/mobile-first-indexing.html)
Why do you (as a business owner) care?
Well if you’re using a mobile responsive website you have nothing to worry about, because the content on your mobile site and your desktop site is the same.
If you serve a different site to mobile users than to your desktop users, well then you need to make some changes. Most importantly, you need to make sure you are using content that isn’t crazy different from your desktop version, because you will be giving your visitors an unexpected user experience when they get to your site. In the long run, if there’s anything that Google will consistently be working to penalize search engine page rank efforts for, it will be a bad user experience.
Typically though, if there is a difference between your desktop and mobile website versions there is probably less content on the mobile. Therefore, the more likely scenario is that you will have a lower search engine rank. Which if is understandable because if your mobile site is showing up for information that only shows on your desktop website and doesn’t show on your mobile site, Google would clearly want to show a different result.
The key, is that they are not using two different algorithms — one for mobile, one for desktop — they will be showing desktop users websites that have good quality mobile content.