@HanasonsAug 03.2013 — #Spam links are generally on unrelated sites, spam quality sites, duplicate content sites.. and those sites which has already lots of links, promoting sex products, viagra, pay day loan etc.
@pintuAug 23.2013 — #spam links are those which are irrelevant and some kind of adult content especially these are called spam links and also low quality content is also considered as spam as per my knowledge am not sure on this.
@saktoriaAug 23.2013 — #It is very difficult to point out which link is spam . As per my experiences a link to your website from irrelevant resources that shares the PR of website is Spam link .
@priyanka444Sep 02.2013 — #if we are submitting our url to a irrelevant site as well as we are getting the back links from the spam site which means the back link has no positive effect towards our site then we can call it as a spam link.. you can simply check it online.
@jessicarobinSep 19.2013 — #Google Webmaster tools help to identify the back links to your website. If your website decrease SERP rankings first check the recent back links and identify whether is it good (relevant) or irrelevant (bad) seo link. Spam links can be removed by using Google webmaster disavow tool.
@alice2691Sep 19.2013 — #That's nice. But i just want to say prevention is better than cure. So, it is better to build links by checking high quality sites that has good PR which may reduce your number of low quality backlinks.
In case you got bad links, you may request for reconsideration of your website after finding and removing the low quality links but its a tricky task to compromise the search engine.