I have seen over and over on this forum how there are "good" and "bad" back links for getting ranked. How can a back link be bad? A link is a link, and just because the linked website or webpage might not correspond to the niche of the other website or webpage doesn't mean it should be considered bad. Why are they negatively looked upon by Google and the other search engines?
back link which are coming to your site should be from High PR sites and Do follow then only your sites strength will be increase.. if you get lot of back link from low PR and irrelevant sites then it will not help you out to Get Good PR and Traffic..
Back Link from high page rank site will help you gain serps and negative back link from spamy or adults site will have bad effect for your rank
Because Google gives more authority to some links over others. If all links were equal, people would just spam their link everywhere (wait, they still do that anyway...) I wouldn't say any link to your website is bad. No links to your website is bad. So a completely unrelated link on a crummy site is definitely a bit less than bad. But there is most definitely good links, very good links, and awesome links out there, and those are the ones you should spend your time on.
Imagine this scenario: you get in friendship with a new guy from a different town..you think he is a gud guy but later your find out that he is involved in doing some illegal activities....so now you call it a bad link.. very similarly in website terms, imagine a website you provide a link to is invovled in illegal activities like (spammin,link spam,pronography..etc..etc)..that is called a bad link... but a link form that domain doesn't count since it is easily possible by anyone to submit your link to that site and get a link from them (could be your competitors)... but if you link back it is a BAD BACK LINK...
Competitors have been known to spam links to rival sites from these bad sources. And yes, sometimes it does work. It's called google bowling.
Backlinks are never negatively looked upon by Search engines. Whatever the worst scenario maybe, all that can happen to you is receive no Link Juice. A 'bad' backlink as you mention could be from a bad neighborhood, adult sites, etc.. not from unrelated sites though. 1 thing you must consider is that you shouldn't ever link to a bad neighborhood. That is something than could get you penalised. This is the main reason why many PLatforms have their comment links as nofollow.
Building backlinks is not that hard but gain quality backlinks is little tough but if your get quality backlinks from relevant sites then it will be very beneficial to the site and also build a brand reputation of the site too.
I m also agree with janny.. when you have good and high Pr back links then it is useful to increase the visitors and also Pr.. so always get back links from high PR sites..