The Tale of Years (Chronology of the Westlands) is said to have been put together at the Great Smials (the ancestral mansion of the Took family) with the aid of material collected by Merry, who in turn gathered much of his information from Rivendell. Tolkien mentioned some of this in his prologue to The Lord of the Rings, and now he offers it here as a mini data dump. It’s a fool’s errand to even attempt to summarize what’s in this beyond the barest of skeletons. If you want the details, and if you’re reading this I assume you do, you really should read it for yourself if you haven’t already.
Essentially, this section is the chronology of Middle-Earth from the Second Age, through the Third Age, and into the early Fourth Age. Most of it is offered as a timeline breakdown by year.
Of the little-documented Second Age, this deals with Númenór and the Rings of Power.
The section on The Third Age deals with years 2 – 3017, focusing primarily on Gondor, Arnor, and The Shire.
The Great Years covers T. A. 3018 – 3019, the time period of The Lord of the Rings, beginning with Gandalf’s return to The Shire and ending with the Ring’s destruction. Battles of the Northern campaign during the War of the Ring that are not covered in the main narrative are touched upon here.
The Chief Days from the Fall of Barad-dûr to the End of the Third Age details the ending of the primary narrative of LOTR, offering up dates through Sam’s return to Bag End.
Later Events Concerning the Members of the Fellowship of the Ring covers the early Fourth Age. All dates are Shire-reckoning as the events primarily deal with the Hobbits. The final entry tells of the fate of Aragorn, Legolas, and Gimli.