Samson is unlocked when the player defeats two floors in a row without taking any damage.
Samson starts with 3 Red Heart containers and Bloody Lust. Every time Samson is dealt damage, he receives a damage boost that resets when he goes down a floor.
Items relying on contact damage like The Virus or E Coli can be beneficial in that you "want" to take damage because of Bloody Lust.
Health regeneration items like Yum Heart, Pyromaniac, and Placenta allow health regeneration after taking damage, and thus allow for more damage, both stat-wise and health-wise.
Cancer and The Wafer minimize damage taken from Sacrifice Rooms and can be incredibly useful to build a lot of damage and pickups, especially in combination with an excess of hearts.
Dead Bird, Best Bud, Smart Fly, etc: All of these items will add to Samson's damage increase from getting hurt, although, unlike Bloody Lust, the bonus of these items only last for the duration of a room.
Fanny Pack's chance of dropping a heart upon hit can cancel out damage to Samson while also buffing him.
Dull Razor can safely provide Samson's damage boost without needing to take damage.
Gimpy increases benefits from taking damage and feasibly adds more survivability from random soul hearts and half-heart drops.
In the Bible, Samson was a supernaturally strong warrior who derived his power from his Nazarite vow, entitling immense strength to Samson if he did not cut his hair.
Samson first appeared in the Binding of Isaac: Wrath of the Lamb expansion. That version had only 1 heart container and a soul heart for his starting health, and his Bloody Lust item acted like the Lusty Blood item, re-added in Afterbirth, though much weaker.
Samson was originally unlocked by skipping two Treasure Room items and killing Mom in one run.