On April 24, 2012 many started to see that Google has started to penalize companies that are buying links for the purpose of passing off the rank. The Google Update was called Penguin. Since then, there have been several different Penguin/Panda updates rolled out by Google. SEM has, however, nothing to do with link buying and focuses on organic SEO and PPC management. As of October 20, 2014 Google has released three official revisions of their Penguin Update.
Alt text (alternative text), also known as "alt attributes" describe the appearance and function of an image on a page. Alt text uses: 1. Adding alternative text to photos is first and foremost a principle of web accessibility. Visually impaired users using screen readers will be read an alt attribute to better understand an on-page image. 2. Alt tags will be displayed in place of an image if an image file cannot be loaded. 3. Alt tags provide better image context/descriptions to search engine crawlers, helping them to index an image properly.
Most people search on mobile devices – You don't need statistics to show you that in the past few years the online mobile market has exploded, overtaking desktops years ago. Optimizing websites for mobile browsers is critical if you want to rank well in search engine results pages. If you’re unsure how your website measures up, enter your site’s URL in Google's Mobile-Friendly Test.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.