Bushido, the art of the samurai, technically is not spellcasting. The abilities are sort of a cross between spells and weapon abilities.
To use samurai abilities you must have a book of bushido. Books are available from samurai trainer in Zento, and come complete with all of the abilities.
Before training bushido to any level at all, be certain that you are willing to accept the consequences. Having the bushido skill will drastically affect your combat strategy regarding parrying:
If you have 120 parrying and 0 bushido, you will parry at a 33% rate when wearing a shield, and a 15% rate without a shield.
If you have 120 parrying and 120 bushido, you will never parry when wearing a shield. However you will parry at a 36% rate when wielding a two-handed weapon and a 30% rate when wielding a one-handed weapon.
Thus it may seem that it is advantageous to use bushido to gain the benefit of increased parrying rates. However wearing a shield becomes a handicap, and this makes it impractical to use any magical benefit from shields, such as any hit chance, defense chance, or other magical properties that a shield may have.
Having the bushido skill has two other effects that are not activated by the book of bushido:
Whirlwind: Whirlwind attacks are more powerful when you have high bushido skill, especially if there are many targets in your whirlwind.
Perfection: Samurai receive damage bonuses when using the honor virtue. Activate the honor virtue from the virtue menu and target a monster, then let the monster attack you. As you defend yourself from attacks, you gain in perfection, increasing damage as you near perfection.
In addition to these benefits, there are a variety of weapon abilities which are not usable unless you have bushido (or ninjitsu) skills, mainly those found in the samurai and ninja weapons.
The samurai abilities which are activated by the book of bushido are described here. There are a few differences on ABC:
Bushido abilities work with melee weapons only, not bows or throwing weapons.
Lightning Strike - There are no critical hits in PVP.
Whirlwind - The damage bonus for whirlwind is not as large as on official shards.
Perfection - There is no luck bonus gained when you reach maximum perfection.
Here are the abilities you should use at each skill level:
60-100: Lightning Strike
100-120: Momentum Strike