From 1d4chan

Weeaboo Fightan monks. They have magic martial arts and swords, and are the biggest weeaboos of all, not to be confused with SWORDMAGE, which is a fighter with a magic sword who totally isn't weeaboo because he's in the Forgotten Realms book rather than the Book Which Won't Be Named

Okay in actuality they rely on "maneuvers" (read: Martial Magic, hence weaboo status) that they prepare, each with a different effect. Of the three base class initiators, Swordsage has the most disiciplines with explicitly supernatural effects allowing them to walk through walls, rip people in half, and even straight up teleport.

In 3.5 they were considered generalists like Binders; instead of having a class wide gimmick, they rely on their maneuvers to be useful. They have good Reflex and Will saves, light armour proficiency, and 6 skill points per level. Speaking of maneuvers, the Swordsage gets more maneuvers, stances, and disciplines than the Crusader or Warblade, with them more or less getting 3 disciplines to themselves, all supernatural: Setting Sun (focuses on throws and counters), Shadow Hand (lets them be better than the Rogue in every way, just by entering a single stance) and Desert Wind (FIRE). They get Discipline Focus as they level up, which lets them add their Wisdom modifier to damage while using a Strike from that discipline, or a +2 to AC while in a stance from that discipline. They also gain the Weapon Focus feat for free, a bonus to initiative, the ability to take no damage on a successful Reflex save, eventually getting the ability to halve damage on a failed Reflex save, the ability to look at a magic weapon or armour and deduce its properties, and finally, the ability to activate 2 boosts at the same time.

Swordsage has two variants suggested in their home book. Unarmed Swordsage loses light armor in exchange for the Monk's unarmed strike progression. This variant is pretty accepted, and became a bit of a meme in the 3E fandom for being a better monk than monk, albeit still needing some common-sense fixes like Improved Unarmed Strike as a bonus feat and the Wisdom bonus to AC working unarmoured. Arcane Swordsage is a vague idea of allowing spells in place of maneuvers, which is just broken.

Dungeons & Dragons 3rd Edition Classes
Player's Handbook: Barbarian - Bard - Cleric - Druid - Fighter - Monk
Paladin - Ranger - Rogue - Sorcerer - Wizard
Player's Handbook II: Beguiler - Dragon Shaman - Duskblade - Knight
Complete Adventurer: Exemplar - Ninja - Scout - Spellthief
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Complete Divine: Favored Soul - Shugenja - Spirit Shaman
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Dragon Compendium: Battle Dancer - Death Master - Jester
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Dragon Magazine: Sha'ir - Deathwalker - Fleshcrafter - Soul Reaper
Dragon Magic: Dragonfire Adept
Dungeonscape: Factotum
Eberron Campaign Setting: Artificer
Heroes of Horror: Archivist - Dread Necromancer
Magic of Incarnum: Incarnate - Soulborn - Totemist
Miniatures Handbook: Favored Soul - Healer - Marshal - Warmage
Ghostwalk: Eidolon (Eidoloncer)
Oriental Adventures: Samurai - Shaman - Shugenja - Sohei - Wu jen
Psionics Handbook: Psion - Psychic Warrior - Soulknife - Wilder
Tome of Battle: Crusader - Swordsage - Warblade
Tome of Magic: Binder - Shadowcaster - Truenamer
War of the Lance: Master
NPC Classes: Adept - Aristocrat - Commoner - Expert - Magewright - Warrior
Second Party: Mariner - Mystic - Noble - Prophet
Class-related things: Favored Class - Gestalt character - Multiclassing
Prestige Class - Variant Classes - Epic Levels - Racial Paragon Classes