The Lord of the Rings are exceptional films. There's no denying that. But as is often the case, the books go into far more detail regarding personal histories, lore, familial relationships, and all other supplementary material of that nature.
When it comes to adapting novels, filmmakers typically get to the nitty gritty and adapt the story and plot - there's simply no time to go into detail regarding Frodo's fifth cousin twice removed on his mother's side. As such, movie viewers miss out on a lot of good stuff regarding series protagonist Frodo Baggins.
Frodo Was Raised In Brandy Hall
Frodo was not actually from the Shire, but nearby Buckland. The location lies between the Brandywine River and the Old Forest. Frodo was raised in a house in Buckland called Brandy Hall, which was built by old Gorhendad Oldbuck. That's because Frodo is descended from the Brandybuck clan, as his mother, Primula, was a Brandybuck. She married Drogo Baggins, and Frodo proved their only child.
His Parents Died When He Was 12
Unfortunately, the Brandywine River brought incredible tragedy to young Frodo Baggins. When Frodo was just twelve years old, his parents - Primula Brandybuck and Drogo Baggins - drowned in the Brandywine River while out boating, making Frodo an orphan. He was subsequently sent to Bag End to live with Bilbo Baggins, who adopted Frodo and took him in as his own.
He Is A Cousin Of Bilbo, Not His Nephew
Even though Frodo refers to Bilbo as his "uncle," Bilbo is actually his cousin. Second cousin once removed to be precise. Bilbo is Drogo Baggins' second cousin, and both Bilbo and Frodo share a distant ancestor named Balbo Baggins. Bilbo is descended from Balbo's son Mungo, and Frodo is descended from his fourth son, Largo.
Lifelong Friends With Merry And Pippin
The films don't really go into detail regarding the relationship between Frodo, Merry, and Pippin.
But in the novels, it is established that they have been friends for a very long time. It's implied that they had been friends even before the deaths of Frodo's parents, and they were known as local troublemakers - often getting into mischief and stealing food (particularly mushrooms) from Farmer Maggot.
Frodo Kept The Ring Hidden For 17 Years
It's never explicitly stated in the film, but Frodo kept the ring hidden for years after being told by Gandalf to keep it safe. If anything, the film makes it seem like no time at all. In the novel, Gandalf's journey is given far more detail, including his trip to Minas Tirith and his research into the One Ring. It takes him seventeen years to return to The Shire and Frodo, who had kept the ring hidden in Bag End.
He Acted As Deputy Mayor Of The Shire
Much of Frodo's story is faithfully adapted throughout The Lord of the Rings, but when it comes to the ending, the novel goes into far greater detail than the film. Frodo is actually quite involved in The Shire after returning from his quest, and even served as Deputy Mayor of the Shire for roughly six months. During this time, The Shire's real mayor, Will Whitfoot, was recovering from an extended imprisonment under Saruman.
Battle Of Bywater
Movie viewers wouldn't know anything about Saruman and The Shire, as that entire section had been removed from the film.
At the end of Return of the King, Frodo and the hobbits return home to find The Shire under the control of Saruman. It has been turned into an industrialized zone, and many of its citizens were cowering in fear and paranoia. Frodo in turn led the Battle of Bywater, which is considered the last battle of the War of the Ring.
He Lived With Sam & Rosie
After returning to The Shire, Sam marries Rosie Cotton (as is seen in the film), and the two have a daughter named Elanor. They proceeded to move in with Frodo, and the three lived together in Bag End together. Frodo was even present for the birth of Elanor. Frodo left Bag End to the Gamgee family following his departure to the Gray Havens, and it remained in the family for at least three more generations.
Frodo Was Eventually Reunited With Sam
The Return of the King depicts a very bittersweet and beautiful ending. The world is saved, but the Fellowship is disbanded and Frodo leaves Middle-earth behind to sail for the Undying Lands to live out his life in pain-free peace. But it's touching to know that Sam, being a ring bearer himself, was also granted access to the Undying Lands, and he eventually reunited with Frodo. They lived out the remainder of their days together in peace. It's also interesting to note that both Legolas and Gimli were granted access as well, but both Frodo and Sam had likely passed away by that point.
Frodo Of The Nine Fingers
Frodo was given many nicknames upon his heroic destruction of the Ring, including Elf-friend and Bearer of the One Ring. But perhaps his greatest nickname has to be Frodo of the Nine Fingers. It's a little cheeky but also respectable, as it honors Frodo's personal sacrifice in destroying the One Ring. For those who don't remember, Frodo was left with nine fingers after Gollum bit one off to claim the Ring for himself.