NRaas is here, the link is also in my signature line: http://nraas.wikispaces.com
(Yes we know that Wikispaces is shutting down later this year, but we will have moved to our new site by then. For now, it's still business as usual over there so the warnings can be dismissed/ignored)
Getting started with NRaas mods in general: http://nraas.wikispaces.com/Getting%20Started%20Using%20NRAAS%20Mods
StoryProgression is by far the most complicated of the mods to get a handle on using and not necessarily the best one to start out with. But with one or two possible exceptions, the default settings it comes with are fine for most players. If you already have a long-running game going, you might want to try SP out on a new test game to see how you like it. But either way, it's totally removable.
Here's what it does and how it works: http://nraas.wikispaces.com/StoryProgression%20FAQ
(skip past the 17 internal links to get to the readable part)
MasterController is arguably a better mod to start out with because other than load its menus on sims, lots, in-game computers, and City Hall, it sits back doing absolutely nothing on its own until the player selects a command to run. SP, with its progression activated, runs all the time, pushing inactive sims around and setting limits for all sims' behavior. That's the one that controls immigration/emigration (culling).
And then of course we have mods like Overwatch and ErrorTrap that don't require any user intervention (OW does have options if one wants to adjust them) but work to keep the game clear of behind the scenes clutter, fix glitches, and keep things running more smoothly.
Edit: Forgot to mention. TS3 mods are always package based content. One must have the Framework set up to work with package files, if you don't already have this.
http://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files