SEM is a broader term than SEO, and is used to encompass different options available to use a search engine’s technology, including paid ads. SEM is often used to describe acts associated with researching, submitting and positioning a website within search engines. It includes things such as search engine optimization, paid listings and other search-engine related services and functions that will increase exposure and traffic to your Web site.
QUOTE: “Another problem we were having was an issue with quality and this was particularly bad (we think of it as around 2008 2009 to 2011) we were getting lots of complaints about low-quality content and they were right. We were seeing the same low-quality thing but our relevance metrics kept going up and that’s because the low-quality pages can be very relevant. This is basically the definition of a content farm in our in our vision of the world so we thought we were doing great our numbers were saying we were doing great and we were delivering a terrible user experience and turned out we weren’t measuring what we needed to so what we ended up doing was defining an explicit quality metric which got directly at the issue of quality it’s not the same as relevance …. and it enabled us to develop quality related signals separate from relevant signals and really improve them independently so when the metrics missed something what ranking engineers need to do is fix the rating guidelines… or develop new metrics.” Paul Haahr, Google 2016
The basics of GOOD SEO hasn’t changed for years – though effectiveness of particular elements has certainly narrowed or changed in type of usefulness – you should still be focusing on building a simple site using VERY simple SEO best practices – don’t sweat the small stuff, while all-the-time paying attention to the important stuff – add plenty of unique PAGE TITLES and plenty of new ORIGINAL CONTENT. Understand how Google SEES your website. CRAWL it, like Google does, with (for example) Screaming Frog SEO spider, and fix malformed links or things that result in server errors (500), broken links (400+) and unnecessary redirects (300+). Each page you want in Google should serve a 200 OK header message.
Because someone who is looking for something that specific is probably a much more qualified searcher for your product or service (presuming you're in the blogging space) than someone looking for something really generic. And because long-tail keywords tend to be more specific, it's usually easier to tell what people who search for those keywords are really looking for. Someone searching for the head term "blogging," on the other hand, could be searching it for a whole host of reasons unrelated to your business.
QUOTE: “When our quality algorithms go to your website, and they see that there’s some good content here on this page, but there’s some really bad or kind of low quality content on the bottom part of the page, then we kind of have to make a judgment call on these pages themselves and say, well, some good, some bad. Is this overwhelmingly bad? Is this overwhelmingly good? Where do we draw the line?” John Mueller, Google 2016
The SEO starter guide describes much of what your SEO will do for you. Although you don't need to know this guide well yourself if you're hiring a professional to do the work for you, it is useful to be familiar with these techniques, so that you can be aware if an SEO wants to use a technique that is not recommended or, worse, strongly discouraged.