Universal results are Google’s method of incorporating results from its other vertical columns, like Google Images and Google News, into the search results. A common example of universal results are Google’s featured snippets, which deliver an answer in a box at the top of the page, so users ideally don’t have to click into any organic results. Image results and news results are other examples.
Every SERP is unique, even for search queries performed on the same search engine using the same keywords or search queries. This is because virtually all search engines customize the experience for their users by presenting results based on a wide range of factors beyond their search terms, such as the user’s physical location, browsing history, and social settings. Two SERPs may appear identical, and contain many of the same results, but will often feature subtle differences.
Another excellent guide is Google’s “Search Engine Optimization Starter Guide.” This is a free PDF download that covers basic tips that Google provides to its own employees on how to get listed. You’ll find it here. Also well worth checking out is Moz’s “Beginner’s Guide To SEO,” which you’ll find here, and the SEO Success Pyramid from Small Business Search Marketing.