The story of My Hero Academia is set many decades in the future when superpowered Quirks have become the new norm all over the globe. Thousands of unique Quirks exist today, but two particular Quirks stand out as the most powerful and the most historically significant of all: One For All and All For One.
These are two of the oldest Quirks to exist in My Hero Academia, and they are at once counterparts and enemies, destined to fight one another someday for the fate of the whole world. By now, many fascinating details have been shed on how both of these Quirks operate and what they're capable of.
One For All: The Power Of Hope & Trust
One For All is the younger "brother" of its creator, the mighty All For One Quirk. What's most remarkable about One For All is that it was never a naturally-occurring Quirk, and thus it has no Quirk factor either. Despite being an artificially created Quirk, One For All ranks among the most powerful Quirks in the world, and it's the only Quirk that the villain All For One truly fears. One For All was actually created by accident when All For One used his own Quirk to transfer a power-storing Quirk to his younger brother Yoichi Shigaraki to help him survive with a frail body. This combined with Yoichi's Quirk, which All For One was unaware of, and One For All was born. Yoichi's original Quirk had no effect except that it could be willingly passed to other people. Although meaningless in itself, when combined with that power-storage Quirk, it became far more.
One For All is unique in that it cannot be stolen by any other Quirks or by mad science. Instead, One For All can only be willingly passed from one wielder to another as a gesture of trust and respect. One For All has had nine total wielders, from Yoichi to Izuku Midoriya, and it has only gotten stronger during that time. What's more, each wielder can imprint their own Quirk factor into One For All, allowing future users to interact with the previous user's memories and personality through that imprint. Izuku saw these imprinted souls during his battle with Hitoshi Shinso in the U.A. sports festival, for example, and he later started using the previous users' Quirks as well, such as Blackwhip.
One For All is, above all else, a melee Quirk, and the user can charge up incredible power and release it with punches and even finger flicks, as Izuku has often proved. The user can also perform Smash attacks to blow away their enemies, and Izuku can spread One For All evenly across his body to use Full Cowling and Shoot Style. However, this comes at a price. A person's body is greatly strained by wielding One For All and their own natural Quirk at the same time, shortening their lifespan. Izuku and All Might were exceptions, having been born Quirkless.
All For One: The Power Of Greed & Exploitation
This Quirk, wielded by the supervillain of the same name, is a meta-Quirk that can transfer Quirks between people, including All For One's user. In theory, this Quirk can be used in benign ways to help rearrange people's Quirks to suit their needs and lifestyles, but the villainous All For One took a different route. He rearranged people's Quirks to earn their loyalty and create an army of followers, and through them, rule the world. Most notably, All For One has spent decades stealing other people's Quirks for himself, amassing a formidable collection of ill-gotten Quirks in his body. Many of those stolen Quirks are best used in combat, but All For One also stole a duplicate of Dr. Garaki's longevity Quirk.
The Quirk All For One has done only one good deed -- accidentally creating One For All. Otherwise, it's the ultimate symbol of theft and exploitation, since its wielder can steal other people's Quirks against their will. Unlike One For All, which is based on trust and generosity, All For One's themes are greed and exploitation, the ultimate symbol of evil. That reflects a broader trend of good vs. evil in fiction -- how heroes work hard to earn their power the noble way, while villains use schemes, deception, theft and cruelty to gain power in unthinkable ways.
Notably, copies of All For One exist, including the miniature version that the movie villain Nine wielded and the copy that Tomura Shigaraki wields. In that sense, All For One is similar to One For All after all, being not just a Quirk, but also a legacy and an idea that can be passed from one wielder to another.