Not to be confused with the Crusaders of the 41st millenium.
A crusader is basically a fall-proof paladin. They use Weeaboo Fightan Magics instead of divine power to kill stuff, which may or may not be better than normal. The crusader also has the power to ignore wounds through adrenaline for a turn and get healing before his body fails from the injuries. Balancing this is the fact that they can't really choose the maneuvers they want to use at a given moment; they just get them at random each turn.
Crusader is the only class (except for the Jade Phoenix Mage and Immortal Blade prestige classes) that gets access to the Devoted Spirit school, where everything is based on what alignment extreme you are and if the opponent just happens to not be the same as you. Yep, totally different from a paladin.
In Pathfinder Crusader is a Cleric archetype. It trades a domain and a large chunk of spellcasting (the entire reason you're playing a Cleric) for a selection of mediocre bonus feats. Even a battle cleric is worse off for taking this over normal cleric.