Taijutsu is a method of using the body for self-defense. Actually, taijutsu is the collective name for any martial art techniques that rely on body dynamics. In some lines of aikido the word taijutsu denotes all aikido work without weapons.

Taijutsu techniques may include strikes, kicks, joint locks, throws and many of the techniques found in the martial arts like aikido, judo and karate, etc. While most of its aspects appear external, the dedicated student will find many internal aspects as well.

Taijutsu is commonly associated with the schools that supposedly teach ninjutsu, although there are many koryu schools with no relation to ninjutsu that have taijutsu in their curriculum, for instance Asayama Ichiden ryu. Taijutsu was used by bushi, some ninjas also used their own version of taijutsu.

As an example, here's a excerpt from the Bujinkan curriculum:

• Taijutsu (unarmed self-protection)
  • junan taiso (body conditioning)
  • taihenjutsu (body movement)
    • ukemi (breakfalls)
    • kaiten (rolling)
    • tobi (leaping)
  • dakentaijutsu (striking methods)
    • koppojutsu (bone attacks)
    • koshijutsu (muscle attacks)
  • jutaijutsu (grappling methods)
    • nage (throws)
    • hajutsu (escapes)
    • gyakuwaza (locks and controls)
    • shimewaza (chokes)