The word spam mostly refers to unwanted, irrelevant content. Mostly black hat SEO’s try on spam Google’s index to increase their web page rank. One should take precaution in creating a blog and writing relevant about the subject, they prefer.
Why people spam?
People looking for short term results spam for link popularity. They are crazy and think it helps in ranking on Google. But actually it doesn’t.
There is nothing wrong in posting link, in your comments, any thing else. Mostly, if you are posting content for user,every thing is OK. For example more comments on a topic means, people are naturally mingling, but spammers come only with the purpose of increasing comment count number, which is indeed a spam.