However, you may encounter pages with a large amount of spammed forum discussions or spammed user comments. We’ll consider a comment or forum discussion to be “spammed” if someone posts unrelated comments which are not intended to help other users, but rather to advertise a product or create a link to a website. Frequently these comments are posted by a “bot” rather than a real person. Spammed comments are easy to recognize. They may include Ads, download, or other links, or sometimes just short strings of text unrelated to the topic, such as “Good,” “Hello,” “I’m new here,” “How are you today,” etc. Webmasters should find and remove this content because it is a bad user experience.
Redirecting is the act of sending a user to a different URL than the one initially requested. There are many good reasons to redirect from one URL to another, for example, when a website moves to a new address. However, some redirects are designed to deceive search engines and users. These are a very poor user experience, and users may feel tricked or confused. We will call these “sneaky redirects.” Sneaky redirects are deceptive and should be rated Lowest.
Since heading tags typically make text contained in them larger than normal text on the page, this is a visual cue to users that this text is important and could help them understand something about the type of content underneath the heading text. Multiple heading sizes used in order create a hierarchical structure for your content, making it easier for users to navigate through your document.

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. 
The errors in technical SEO are often not obvious, and therefore one of the most popular. Mistakes in robots.txt and 404 pages, pagination and canonical URLs, hreflang tags and 301 redirects, http vs https and www vs non www versions: each of them can seriously spoil all efforts to promote the site. One quality SEO website analysis is enough to solve all the main problems in this part forever.
When would this be useful? If your site has a blog with public commenting turned on, links within those comments could pass your reputation to pages that you may not be comfortable vouching for. Blog comment areas on pages are highly susceptible to comment spam. Nofollowing these user-added links ensures that you're not giving your page's hard-earned reputation to a spammy site.
Another illicit practice is to place "doorway" pages loaded with keywords on the client's site somewhere. The SEO promises this will make the page more relevant for more queries. This is inherently false since individual pages are rarely relevant for a wide range of keywords. More insidious, however, is that these doorway pages often contain hidden links to the SEO's other clients as well. Such doorway pages drain away the link popularity of a site and route it to the SEO and its other clients, which may include sites with unsavory or illegal content.