SPOILERS! As we explore the past and as the seasons progress, more will be revealed.
Unless you've read A Song of Ice and Fire, you probably aren't totally sure what's going on all the time on Game of Thrones, or what in the Seven hells some of the characters are talking about when they discuss historical events. Even if you read the books very carefully, you can still find yourself reeling - Game of Thrones history is just that dense.
For example, did you know that Robert Baratheon and Ned Stark grew up together as wards of Jon Arryn? Do you know why Daenerys named one of her dragons Rhaegal? Do you know who broke up Tyrion's first marriage? And why does Littlefinger seem to hate everybody? All of these tidbits come from events that occurred before the show even starts!
Fear not! This list is here to quickly fill you in on the most important backstories that inform the events of Game of Thrones. Some are very recent, while others go back 8,000 years. Since we all know that history repeats itself, cast your votes for which events will be the most pertinent and influential moving forward in the new seasons.
Lyanna Stark Was Kidnapped/Ran Away with Rhaegar Targaryen
During the reign of Aerys II Targaryen, Lyanna Stark was betrothed to Robert Baratheon. She was kidnapped by Prince Rhaegar Targaryen - which, depending on whom you ask, was either very romantic or very rapey. It should be noted that Rhaegar was already married at the time. Lyanna's father, Rickard, and older brother, Brandon, were executed without trial in a horrible, torturous way by the Mad King after they tried to rescue her. This ignited Robert's Rebellion.
Lyanna died in Dorne in the Tower of Joy after making her brother Eddard swear a secret promise. We now know from the Season 6 finale that Lyanna made Ned promise to keep her son's identity a secret. Why? Well, if you believe in editing and dolly shots, Jon Snow is the son of Lyanna and Rhaegar (a theory often referred to in shorthand as "R+L=J"). She made Ned swear to protect Jon because Robert would have had Jon killed and Ned would have to fight his childhood friend and king.
The Wall Was Built to Keep Out Frozen Death
When you crack open a book or start the show, the first thing you're going to learn about is the Wall. A 300-mile-long preventative measure, the Wall was built 8,000 years before A Song of Ice and Fire. It separates the pleasant and habitable southern half of Westeros from the hellish northern tundra where the wildlings are, as well as the White Walkers.
The Wall is more than just a giant wall of ice, however. It was also warded, meaning that magic was embedded in the Wall to repel White Walkers.
It also separates the normal folk from spooky supernatural monster sh*t, but when the story begins, that is all taken for granted. Since Westeros hasn't seen the "Others" (aka, the White Walkers) and their army of undead wights for several generations, they've practically been forgotten.
Attacks by the Walkers seem to coincide with Westeros's ridiculously slow-progressing seasons and now, at last, "winter is coming," so... it's kind of a problem that the Wall is now understaffed, under-equipped, and under-provisioned. It's also manned exclusively by criminals, bastards, and other undesirables.
Oh, and Bran has been marked by the Night King, which could very possibly unward the Wall and invite the undead army right into the south and from there, the rest of the world. There's a very good chance that Bran will be a part of bringing down the Wall or at least a piece of it.
Robert Had a Rebellion of His Own
Robert's Rebellion is probably the most pressing matter facing the world of Westeros when the first book in the series, A Game of Thrones, begins. The rebellion happened for a couple of reasons:
1. The Targaryen crown prince, Rhaegar, kidnapped (or perhaps, ran away with) Robert Baratheon's betrothed, Lyanna Stark, which peeved the North, the Stormlands, and fans of not-kidnapping-people everywhere.
2. The reigning king, Aerys II Targaryen, had gone quite mad and developed a sick affinity for fire and burning people. He was teetering on the edge of civil war with his own son Rhaegar, the region of Dorne, and several other paranoid fantasies of his own.
The war was won by the rebel forces, which had some important consequences:
1. Rhaegar died.
2. King Aerys II Targaryen died.
3. Lord Rickard Stark and his son, Brandon, died, making Ned Stark the new Lord of Winterfell.
4. Robert Baratheon didn't die. He became the first non-Targaryen king in nearly three centuries.
5. Jon Arryn became Hand of the King.
6. The Lannisters sacked King's Landing, smashed the infant Targaryen heirs, and expected to be rewarded with the Handship.
7. The Lannisters didn't get the Handship, but they got to marry their oldest daughter, Cersei, to Robert instead.
Tywin Lannister Sacked King's Landing
At the close of Robert's Rebellion, after the Battle of the Trident took his champion and son, King Aerys II Targaryen opened his gates to Tywin Lannister and his army, despite warnings from Jaime and Varys. But the Mad King listened to Grand Maester Pycelle and opened the gates anyway.
That turned out to be bad idea, because Tywin sacked the city. Amidst the chaos, Aerys had a couple of even worse ideas. He ordered the Lord Commander of his Kingsguard, Jaime Lannister, to behead his own father (Tywin). He also ordered his pyromancers to use wildfire (planted throughout King's Landing) to burn the city and everyone in it.
Faced with some tough decisions, Jaime killed King Aerys and his pyromancers instead. He broke his vow to protect the king, but also saved thousands of lives. Sadly for Jaime, not everybody comprehends why he slayed the king. When he entered King's Landing, Eddard Stark found Jaime presumptuously seated on the Iron Throne, and he never seemed to forgive the faux pas.
Meanwhile, Gregor Clegane was busy raping women and also smashing the Targaryen babies (who were also half-Dornish). The whole event was upsetting to many great houses.