There is good and there is bad. There is yin and there is yang. There are good ways to building web pages and there are spammy ways of doing that. I have covered the aspect of quality of content in a different post here. Now let's look into the other way of building pages also known as spammy way. Building spam is about building pages whose sole aim is to get high in the search to drive the traffic their way.
Let's look into various such concepts which come in the category of spamming and may lead to deindexing from search engines or a possible ban. Most of these concept have a useful element in it but with time these were manipulated to rank higher on search results.
In simple terms, link farms are reciprocal links. In a simple situation it will be site A linking to site B and site B linking to site A. In more complex scenario there will be a circle of links from site A to B to C to D and back to A. Link farms become spam farms when the intention of linking is to get inbound links so that the pages can get better page rank. The sites are not linked for content but for links.
There are agencies and companies who are expert in building link farms. Many times it is done my buying an expired domain having high page rank and than using that as the core of link farm, the page ranks are propagated to other web pages.
Linking in the context of content is good and anyway in this case there is usually no backlink. But linking for page rank is heavily discouraged by search engines. Google is known to have come heavily in penalizing link farms and the sites participating in them. Avoid getting into trap of a promise by someone that they will create massive amount of backlinks for you overnight. They are more or less going to put you in a link farm. The initial success may put you in euphoria but soon you will realize that your site has been deindexed or worse permanently banned. By that time the agency would have walked away with their cheque.
Guest Blog and Comment section
This was quite popular at one time but with time this again has become target for spams. People have used this to create thin content or unrelated content to link back to their web page. You have written a post on 'Internet of Things' and someone comes and puts a comment something like
'This is the best post I have seen on this topic. Keep up the good writing. For best golf sticks in the town contact at @@@@@'
People have even written bots which automatically go and put comments on web pages. If you do not have a good captcha type of security in place, soon your page will get filled with unsolicited comments. I have a filter in place so that I can review the comments before approving. At times they are very funny but search engines don't find them funny. Now a days search engine like Google also looks into the surrounding word and context of the page before qualifying a link as inbound link.
Forums and Social sites
Same aspects apply for forums and social sites as for guest blogs and comment sections. Putting links with the intent of linking can put things in trouble. If you are using adsense in your page than you want to be careful about social sites. The bottom line is intent. If the intent of linking is relevant in the context and user will navigate the link for the content, things are fine. If the intent of linking is to create inbound links, soon or later the trouble is going to happen.
Private Blog Network (PBN)
Private blog networks come in the same category as link spams in some way. People make a ring of multiple blogs participating in that network. The network might be set up by one single person or multiple people and link exchange happens in the network. It's a network of blog promoting each other. Lately Google has come heavily on such networks and has banned a lot of websites identified to be participating in such networks.
It's fine to promote your website. But be careful that the paid links should be put as nofollow attribute. For details see this post. For search traffic a nofollow attribute works. This tells the search engine to not to consider the link as an inbound link. However if your are hosting adsense account than the paid links in any form are no no. As paid links might drive traffic which will lead to clicking of ads, this is not considered a good practise.
It's a common practise to buy expired domains with high page rank and redirect them to your web pages. This is done so that the link juice of high page rank can be passed to your web page. Search engines have become smarter and this kind of redirection is looked with suspicion. If the redirection happens for the similar kind of content and keywords, search engine pass the juice. If search engine consider it as linking to get the link juice only, the juice is not passed. So be careful before spending money on buying such expired domains.
At the End
Beware of such link spamming techniques. Someone promising you millions of backlinks overnight is set up to take your money. There is an industry who thrives on building inbound links and creating artificial traffic overnight for short term results. There are people who get paid to click on links to create traffic.
There is no easy way to build ranking apart from doing hard work of writing good quality content and climbing on search rankings organically. If people like your content you will acquire more traffic with time. The best traffic is one that you get by people clicking on search results of search engine and that requires the reputation, which is build with time.