On April 21st 2015, Google introduced its mobile-friendly update.
How are sites affected?
If you are searching for a website on anything but a mobile phone, the update shouldn't affect you at all.
But if you use a mobile phone to search, the search results now boost websites that are mobile-friendly. And this is a big deal.
What needs to be done?
The bottom line is that any website development ought to be mobile-friendly from now on. It's not always essential - if you're really not bothered about people finding your site on a mobile phone - but it is a very good thing to do. It does, however increase the cost of setting up a website.
What are the options?
At its simplest, websites should have a mobile-friendly layout as well as a desktop-friendly version. But to be truly forward thinking, websites should also work on screens that are larger than mobile and smaller than desktop - such a website is called 'responsive'.