In addition to what Chedders as provided you....
Links with the 'No Follow' tag will not hurt your site. You should be building backlihks for traffic and not ranking.
Google now uses the tags, "no follow, sponsered, and ugc", as hints and may in fact count them. The links will not impact your seo in a negative way. I look at it this way, they are mentions and mentions do carry weight. How much weight is up for conjecture.
As Chedders said, stay away from bought / paid links, they just cause trouble in the long run.