A hakama is the skirt-like pants that some Ju-Jitsu practitioners wear. Traditionally, it was worn by the samurai. The standard gi worn in Karate, Aikido and Judo classes, as well as in other martial arts, was usually the underclothes.
Originally, the hakama was worn as an outer garment to protect a samurai horseman's legs from brush, weeds, etc. (similar to a cowboys leather chaps). In Japan, since leather was so very hard to come by, heavy cloth was used in its place. After the samurai made the transition from mounted soldiers to foot soldiers, they continued to wear the hakama largely due to the fact that it set them apart and made them easily identifiable.
Saito Sensei tells the story about hakamas during the old days in Hombu Doj
Most of the students were too poor to buy a hakama but it was required that everyone wear one. If the student could not get one from an older relative, they would remove the cover from an old futon, cut it, dye it, and have it made into a hakama. Since the dyes that were used were very cheap, the colorful pattern of the futon would start to show through the dye. Eventually, the stuffing from the futon would start to work its way out of the material.
The Hombu dojo was a very colorful place, with all the various colors of hakama. Traditional hakama were not just solid colors. The material used often had patterns woven or printed onto them.
In most Ju-Jitsu schools of today, the hakama is reserved for the yudansha (black belt students). Few schools allow all practitioners to wear one, while some schools allow women to start wearing it much earlier than men (general modesty of women is the explanation, since a gi was originally undergarments).
The hakama has 7 folds in it (5 in the front, 2 in the back) and has the following symbolic meaning:
Yuki - courage, valor, bravery
Jin - humanity, charity, benevolence
Gi - justice, righteousness, integrity
Rei - etiquette, courtesy, civility (obedience)
Makoto - sincerity, honesty, reality
Chugi - loyalty, fidelity, devotion
Meiyo - honor, dignity, prestige