If we built 100 backlinks from other sites with pr3+ but the sites were not relavant to your content where you placed. so how much it will be effected by search engine to boost my site's rank?
IF it is not relevant means it will be useful for you only 50%. If it is related to your site means then it is 100% useful to you
I would say that it is even less than 50% useful to you. You need to get links from good and related sites to be of some effect on your rankings.
Getting quality backlinks (I assume your PR3 sites are good sites) to your site is always create positive result in ranking. There is no guarantee in level the benefit as Google only one who can tell it. There are some other factors to consider such as your competitors, your on-page seo , etc. The only way is to know is to test it yourself. Keep tracking over time and every time you add the new backlink to it.
The only backlink that will be beneficial is a quality one. Non related backlinks won't hurt your site but won't benefit either
backlinks can be considered as one way to improve PageRank. supported with good content and interesting.
It's not correct. Non related links can hurt your site. It's just a quantity problem. If you have 1% irrelevant backlinks, those links can be disregarded. But 5% or more of your total backlinks are at some irrelevant pages, those backlinks can be easily considered as paid links. If those backlinks have nofollow, then they will be spam links in the eyes of google. Also the pages which have irrelevant external links are punished if they are dofollow.
Relevancy matters in ranking for high competition keywords. Otherwise, you gain significant SEO effect from all good backlinks, not depending on relevancy.
I have never heard about Google penalty the irrelevant backlinks. Did Matt Cutt say that in public? If there is a proof please share here, so I can destroy my competitor very easily to just send them those backlinks. I could pay a scrapebox boy to spam those link which is cheaper than I building the backlinks to my site. If you do not know anything for sure, so please just don't speak.
Links are links and they will be positive for your site in any case. The only thing it will be negative if it will be placed on shit places with a lot of trash.
I don't agree with you, that does not make sense. So you mean to say that if you have links from unrelated sites then it can hurt you. If that would have been true it would have been very easy to bring your competition down just by linking them from crappy and unrelated sites. Your rankings depend upon who links to you and whom you link. You might be penalized for linking to an unrelated or crappy site but you are not going to be penalized if some crappy site links to you.
@akasheg, it's true. It's a matter of quantity. As for the competitors, it's still a matter of quantity. If you are talking about a site with millions of relevant backlinks, a few bad backlinks can't affect it. But if you have a fresh site, try to buy a cheap PR link on a non-english site or visit other people's blogs to just leave a few comments then watch how google removes your site from the index in the same week.
No one can say exacts words on what will happen but I think results will not be as much good as expected by relevant sites.
Okay say that then even according to your logic its very easy to pull some one down. Consider a site ranking #1 for a keyword not much competed then it would require lesser no. of backlinks to rank high but its very easy to link that site with spammy links you just have to submit to FFA pages (which google does not like) which you can do easily using various tools you can also do the same for blog posting on norelated and spam blogs which is again very easy. As it requires not much of an effort to get links from these websites. I would still say the idea is not correct, and as far as buying the links is concerned Google is completely against them and will surely penalize you if it catches you doing that.
I haven't tried to pull someone else's sites down. I am not discussing this. Because it's not ethical. Try to improve what you own. Never think to harm other people. At the end, you can harm only yourself. Irrelevant backlinks can hurt your site if they are too many for your site. Even relevant backlinks can also hurt your site if they have some similar patterns (comment links, directory links, inline links within content, only banner links, no different anchor, etc.) Search engines analyze each link then they label some links as "natural". If you know which links are "natural" then you can define all the other links as "unnatural". If you know all of the "unnatural" links, you can easily remove all of them from the index. Irrelevancy is one of the most important factor of not labeling as "natural" Search engines have to display spam free results. They have to penalize the sites with too many irrelevant backlinks and too many same/similar backlinks.
I am not saying to harm your competition I am just keeping my view that it is not possible! You harm yourself with whom you link to but you can't get harmed by who links to you, simple as that.
You should build relevant backlinks to get ranked. But if you build irrelevant backlinks then the ranking will depend on the diversity from relevancy. It may be vary form 10% to 50%.
It would be effected some say, but generally it would have to be targetted to mean much for your keywords, but you would still get traffic and go up in the search engines, just not as much as you would have if they were targetted