User blog comment:Commander VB/Mass Effect 3 story understanding/@comment-5337704-20120824131318

What we know:

Citidel is just a mass relay.

Catalyst AI: Was designed to broker peace between the AI and the organics that wanted to destroy the synthetics (original creators vs their synthetic creations). Think of the whole Quarian vs Geth back story here.

Catalyst decides the best course of action on saving organics from complete destruction is hybriding them into synthetic organic creatures, which happen to be what we know as "Reapers".

Relays: Created by either the original race or reapers to keep any future organics to follow the same technolgical line so they can shut down the technology to make reaping easier.

Keepers: To keep organics from discovering the true nature of the citidel, during our time frame to "activate" the catalyst/citidel.

Sovereign: A reaper left behind to signal the keepers when organics reached their apex.

From my understanding, and be kind this is a loose logic on my part:

If the catalyst is the citidel, lets say the catalyst is my Toaster that has a push down button: The toaster cannot press down the button itself, so the first "something "does so, possibly an organic that activated the program, or the first reaper or keeper, who knows... We do know that the keepers were used during the game to push said button to activate the catalyst/citidal. The cruicible was designed to for organics to negotiate with the catalyst, basically, the old "STOP" key on certain computers. The funky bit, is if the crucible itself was really what stopped the catalyst, or if it was the AI programming breaking down because it refused to believe any organics would be able to accomplish it.

Either way, the catalyst is just a running program that needed input when it relized that organics have superceded the need for the cycle (the new organics are powerful/intelligent enough they could actually defeat the synthetics). Of course, if you choose one of the options to defy the catalyst, it figures out Shepard isnt smart enough and continues the cycle.