I've heard a lot about reddit from my friends.  Do you have any idea what it is?
Reddit is basically a collection of user-made boards that cover just about any topic you can think of.
But how does one go about filling in the topics?  If it's so broad, won't most boards be empty and have no content?
The people who fill the sub-reddit are the ones who have an interest in that topic. Some are more popular than others and the amount of content is purely based off the users themselves.
Does it get moderated?  I can see there being a lot of issues with user content if anyone can post anything.  For example, do they have filters to remove non-relevant content or discriminatory content?
Yes, there is some moderation but it is usually the people who start the group that are the moderators. The creators of the site themselves won't do anything because they believe in free speech as long as the content is legal.