How does “Reputation” work?
I was recently awarded +100 reputation by the Stack Overflow site. The message said it was because I passed the 200 reputation cost. Is there a document describing all of the site's trophies?
This question provides a detailed breakdown of the reputation system, including the various ways in which you can gain and lose reputation points. Specifically, note this item:
So you must have associated your account on another Stack Exchange site with your account on Stack Overflow, where you had over 200 reputation points. For doing so, you get a +100 bonus on the site with the linked account.
This is because we assume you're already familiar with the way the Stack Exchange network operates, and we want you to have voting, commenting, and a handful of other basic privileges on the new site to start out with. You'll have to earn the rest by proving yourself knowledgeable in the applicable subject matter!
Note that this is a one-time bonus for each site, and it cannot be taken advantage of twice. Otherwise, this is the only way that your reputation will "automagically" increase without any direct or obvious correlation to actions taken on the site (such as posting, voting, accepting an answer, etc.)