Giants are large, powerful creatures that are native to the continent of Xen'drik. They physically resemble humanoids, though at a much larger scale, with the smallest of giants still standing at about 16 feet tall, whilst the tallest stand about 26 feet tall. They once ruled Xen'drik, but their entire civilisation was destroyed by the dragons of Argonnessen for their abuse of magic.
The first Giant civilisations arose on Xen'drik roughly 30,000 years ago, and they quickly spread out and became the dominant people on the continent. These ancient giants are generally perceived to have been stronger and larger than the modern giant species, which has led to some scholars to refer to these ancient giants as titans to differentiate them from their modern kin. The giant civilisations quickly covered the continent, and they thrived, but at great cost to others. The Giants raided the feyspire of Shae Tirias Tolai, taking the eladrin as slaves, whose descendents would become the modern elves.
Roughly 20,000 years ago, the dragons of Argonnessen reached out to the various giant civilisations and shared with them the gift of arcane magic, an event that they now refer to as kurash Ourelonastrix, or "Aureon's Folly" in common. The giants used this gift to advance their civilisations, and they quickly became masters of the arcane arts, pioneering the art of elemental binding that the Zil use today. The creation forges that were created by House Cannith were based on relics and designs recovered from Giant ruins.
There were multiple Giant civilisations present throughout Xen'drik's history, and scholars disagree over whether they all existed at the same time. The most notable of these were the Cul'sir Dominion, the Sul'at League, and the Group of Eleven. The Cul'sir Dominion is believed to have been the preeminent civilisation on Xen'drik, and the modern city of Stormreach was built over the ruins of the Cul'sir Dominion's capital. The Sul'at League were a group of mages that pioneered the art of elemental binding, and are believed to have created the Drow, whilst the Group of Eleven was an alliance of eleven independent city states that each focused their research on a particular plane of existence.
About 14,000 years ago, the Cul'sir Dominion made contact with Dal Quor, who invaded Xen'drik to try and escape from an oncoming catastrophe. All of the Giant civilisations went to war with the Quori, which ultimately culminated in the creation of a device the Giants referred to as the "Moonbreaker", which they used to shatter the moon Crya and permanently sever Dal Quor from Eberron. The recoil of this action was immense, and plunged the whole of Eberron into chaos for decades, as magic became uncontrollable and the skies rained blood. The elves seized the opportunity and rebelled against their masters, which lead the Sul'at League to create the Drow to try and combat the elves. This new conflict lasted for hundreds of years, before the Giants turned once again to the Moonbreaker to try and end it.
That turned out to be a step too far for the Giants, and the dragons descended upon Xen'drik enmasse. The dragons utterly annihilated the giant civilisations, and placed multiple powerful curses on the continent to ensure that the giant civilisations could never rise again. The creation and usage of the Moonbreaker was deemed to be an unforgivable abuse of the gift of magic that the dragons had entrusted to the giants, and lead to the dragons deciding that mortals could not be trusted with arcane magic. In the midst of this draconic apocalypse, the elves fled to Aerenal.
Today, there are still giants on Xen'drik, but they are a pale shadow of their ancestors, living among the ruins of their once great civilisations. It is unclear when the various subspecies of giant developed, but it is generally agreed upon that the ancient giants were all one species.