Trolling is the behaviour of Trolls. Trolls are users who, reacting to the deindividualisation of the Internet feel secure in their anonymity to attack other posters with impunity.
In real life, being bone cowards/ unable to defend themselves whether they are High School Boys or fat and frustrated middle aged women, the internet becomes a psychodrama where they can be all powerful. To add to their power they create 'duals' that is a number of identities so as to build a Repulsion; that is a 'group' of Trolls which attack others.
Trolling is the behaviour of a troll or a 'Repulsion' where they make unnecessary insulting/attacking comments whether on Message Boards, Blogs, etc. Reddit, for example, is a perfect venue for Trolls. They (or one person with a number of nicks) attacks the shares of another, leaving insulting/attacking/unnecessary comments designed to hurt a person they have never met and know nothing about.
Mild Trolling is the unnecessary word or sentence left as a comment or response. For example; "Gee, you're stupid" without more. Medium Trolling is when this kind of a comment appears frequently. Severe Trolling is when a Troll, with a number of duals goes after a poster, leaving comment after comment, attack after attack.
Hence, returning to the worthless site for 'networking' "Reddit", mild Trolling would be a 'Gee You're Stupid' comment under one's post. Medium would be a number of such remarks under a post. Severe Trolling would be attacking every single Reddit one had posted, making it their virtual reason for being to attack the user.