Reddit is a rather eclectic social media networking website. It describes itself as more of a platform than being one-thing-in-particular. Even so, anyone that makes an account on it becomes a ‘redditor’ which implies some kind of agreement to some basic set of rules. Unfortunately, the basic rules are not really enough to get you very far.
The key to reddit are the subreddits, of which there are many thousands. These are essentially topics that sit underneath the main site (front page). For example reddit.com/r/science deals with science news commentary. A more practical way to using reddit is to subscribe and unsubscribe from various subreddits.
Each of these subreddits has its own specific rules, requirements and expectations. These are usually highlighted on the right hand sidebar of the pages.
What makes reddit popular?
One of the keys is that reddit has a large diverse user base that are able to interact on the site via various means such as up voting, down voting, accumulation of virtual points (Karma) for submission/comments , nested commenting system as well as PM(private message) capacity.
In addition subreddits have moderators and various auto moderating systems to minimise spam or abuse.
How do some submissions get so many comments?
One of the main reasons that some submissions get so many comments is that the commenting system is nested so that a reply can be replied to. The content of the reply often ends up as being a mini conversation between users and the topic may go off on a tangent.
In addition there is a system of "karma" points awarded to people that get a comment up voted (often if the comment is considered insightful, interesting, useful or funny). This can lead to people being motivated by a sense of reward by getting more karma points.
Anonymity is also a feature that give people a slightly greater level of confidence to add witty comments (where allowed).