One of my favorite things about The Force Awakens has to be all the new fans it's brought to the Star Wars franchise. I'm that person at the party who does a double take when someone says they've never seen any of the original movies. Star Wars is for everyone, and I'm so happy to welcome noobs into the club. But people who didn't grow up with Star Wars have a lot of questions, such as, "Are the Storm Troopers clones?" or, "What are midichlorians?" or, "When is Rogue One set?" I'm here to answer them, in a gentle and non-judgmental way.
As for the first two questions, the answers are "only some of them" and "shut up; they don't count." Now that we have those out of the way, I'll break down the basic timeline of Star Wars. Forgive me if this seems patronizing, but first and foremost, I need to point out that the series takes place a long time ago. Yes, those are literally the first words you see on the screen in the first movie, but you'd be surprised how many people gloss over that and assume that Star Wars takes place in the future because they have laser swords. Let's dig in.
Much like our current Western calendar centers around a single date, so too does the Galactic Standard Calendar. All years are referred to as BBY (before the Battle of Yavin) or ABY (after the Battle of Yavin). The Battle of Yavin refers to the events that take place at the end of Episode IV: A New Hope, when (SPOILER) the Rebels blow up the first Death Star. The year that A New Hope took place can be referred to 0 (zero) BBY or 0 ABY interchangeably.
Episode V: The Empire Strikes Back occurred in 3 ABY, or three years after A New Hope, Episode VI: Return of the Jedi took place in 4 ABY, and Episode VII: The Force Awakens took place in 34 ABY. As for the prequels, Episode I: The Phantom Menace (aka Check Out This Annoying Kid) took place in 32 BBY, Episode II: Attack of the Clones (aka Anakin Hates Sand) took place in 22 BBY, and Episode III: Revenge of the Sith (aka Thank The Maker It's Over) took place in 19 BBY. Got it? Good.
A New Hope centers around stolen plans for the Death Star that enable Luke, Leia and the gang to blow it up. But who stole those plans? That would be Jyn Erso, and that's what Rogue One is all about. We know that it takes place sometime before the Battle of Yavin, since those plans are the catalyst for the battle itself, but we still don't know the exact year. Director Gareth Edwards told fans at Star Wars Celebration that Rogue One takes place "closer to Episode IV" and the rumor mill puts it somewhere between 5 BBY and 3 BBY. Why they'd go to all the trouble of stealing the plans and then sit on them for three to five years is beyond me, but then again, I'm not leading a galactic rebellion, so what do I know?
One other thing to note about when Rogue One is set: Fans who have seen all the films (even the bad ones) will recall that, while Jedi were once prolific and well-respected, a certain someone killed most of them in Episode III (19 BBY), and by the time Luke started his Jedi training (0 - 4 BBY), there were only two left, and they both eventually died. That means that there are no Jedi in Rogue One, which will be pretty weird for a Star Wars movie.
But that doesn't mean there's no Force at all; MakingStarWars.net has some OMG-I-hope-it's-true scoops on Darth Vader appearing in Rogue One, and supposedly the original storyboards "showed decapitated rebels, people being dismembered by Vader throwing his sword, levitating bodies to make human shields." Yasssss. Listen, I loved how today's technology made it possible for Kylo Ren's Force powers to be absolutely amazing in The Force Awakens, but at the same time, I did not appreciate that it was Kylo Ren that had them. Darth Vader is the O.G. Star Wars villain. Kylo Ren worships Darth Vader. How does it make sense for Kylo Ren to have better powers than Darth Vader? It sounds like Rogue One is about to solve that problem. Now, we just have to wait eight months to see something that happened a long time ago.