You need to keep an eye upon your backlinks with the help of Google Webmaster Tools and must remove the spammy looking backlinks by using Disavow links tool. The Disavow links tool would discredit those backlinks with your site and keep your site safe on the search engine rankings.
Here is a video from Google’s Webspam Head, Matt Cutts explaining about Negative Seo. Have a look at it!
Also See:-
Google Patent Named Ranking Documents to Penalize Spammers
What is Seo?
Domain authority and Google results
How does Google applies Semantic Search
Emanuel Update
Google Sandbox effect
Google Penguin update
Knowledge Graph
Google’s Take on Sitewide Links
Google Human Quality Raters Do Not Influence A Website Ranking Directly
Google Now Cards
Google Expands Knowledge Graph
Query Highlighting on Google
How to Create Backlinks That Google Loves
Google to Take Action Against Spammy Guest Blogging
Seo Tools