Will someone please explain "trolls" to me?

I mean, I understand the definition as it relates to the blogosphere; I just don’t understand the purpose.  I’m not new to reading blogs and comments; however, I am new to commenting.

I joined the NRCC on Facebook when I got home from the Redstate Gathering.  Just now, I received an update from them on Facebook about “targeting 70 democratic seats.”  This interested me so I followed the link.  The comments are horrible.  There is nothing even remotely intellectual in the comments. 

While I understand the perceived dangers of moderating comments too closely, this is a farce.  Intelligent discussion is one thing but  a pure disruption of the comment section is something else entirely.  This disgusted me. 

So, why does this exist?