Another example when the “nofollow" attribute can come handy are widget links. If you are using a third party's widget to enrich the experience of your site and engage users, check if it contains any links that you did not intend to place on your site along with the widget. Some widgets may add links to your site which are not your editorial choice and contain anchor text that you as a webmaster may not control. If removing such unwanted links from the widget is not possible, you can always disable them with “nofollow" attribute. If you create a widget for functionality or content that you provide, make sure to include the nofollow on links in the default code snippet.
QUOTE: “Over time, we’ve seen sites try to maximize their “search footprint” without adding clear, unique value. These doorway campaigns manifest themselves as pages on a site, as a number of domains, or a combination thereof. To improve the quality of search results for our users, we’ll soon launch a ranking adjustment to better address these types of pages. Sites with large and well-established doorway campaigns might see a broad impact from this change.” Google 2015
What about other search engines that use them? Hang on while I submit my site to those 75,000 engines first [sarcasm!]. Yes, ten years ago early search engines liked looking at your meta-keywords. I’ve seen OPs in forums ponder which is the best way to write these tags – with commas, with spaces, limiting to how many characters. Forget about meta-keyword tags – they are a pointless waste of time and bandwidth.
Webmasters and content providers began optimizing websites for search engines in the mid-1990s, as the first search engines were cataloging the early Web. Initially, all webmasters only needed to submit the address of a page, or URL, to the various engines which would send a "spider" to "crawl" that page, extract links to other pages from it, and return information found on the page to be indexed.[5] The process involves a search engine spider downloading a page and storing it on the search engine's own server. A second program, known as an indexer, extracts information about the page, such as the words it contains, where they are located, and any weight for specific words, as well as all links the page contains. All of this information is then placed into a scheduler for crawling at a later date.
After a while, Google will know about your pages, and keep the ones it deems ‘useful’ – pages with original content, or pages with a lot of links to them. The rest will be de-indexed. Be careful – too many low-quality pages on your site will impact your overall site performance in Google. Google is on record talking about good and bad ratios of quality content to low-quality content.
For example, let’s say you were running the website of an online pet store. You might be wise to create one keyword grouping for all your dog-related products, then one for all of your parakeet-related projects, etc. The next step would be to segment each individual group into smaller subgroups (parakeet cages, parakeet toys, parakeet snacks) and then even smaller groups for each type of product (low-fat parakeet snacks, luxury parakeet snacks… you get the idea). Now your pet store can create individual pages optimized for each small keyword group.
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. 

If you own, manage, monetize, or promote online content via Google Search, this guide is meant for you. You might be the owner of a growing and thriving business, the webmaster of a dozen sites, the SEO specialist in a Web agency or a DIY SEO ninja passionate about the mechanics of Search : this guide is meant for you. If you're interested in having a complete overview of the basics of SEO according to our best practices, you are indeed in the right place. This guide won't provide any secrets that'll automatically rank your site first in Google (sorry!), but following the best practices outlined below will hopefully make it easier for search engines to crawl, index and understand your content.