Quiincy the history:
The Quincy are nearly extinct due to extermination by the shinigami about 200 years before the main Bleach storyline.
As opposed to the shinigami, Quincy actually kill hollows, rather than cleanse their spirits. This difference was what led to the Quincy-shinigami war. The Quincy's actions were slowly disrupting the balance of souls between the two worlds, and would eventually have destroyed both worlds. The end of the war showed the shinigami victorious, and the Quincy line was closely monitored over the decades.
The final point of contrast between Quincy and shinigami (though minor, at best) is their respective choice of uniform. While the shinigami wear traditional, black, flowing hakama, the Quincy wore slightly futuristic, white, and form-fitting western-style clothing. One peculiar recurring motif of Quincy is that of a cross, which appears frequently in their uniforms. The significance of this is unknown.
This, as well as the attitude and goals of the Quincy, draws a comparison from many between the Quincy and the knights of old.
Combat~
Unlike shinigami, who use their inner spiritual power to fight, the Quincy fight using the spiritual power of their surroundings.
The Quincy's weapon is the bow, which they can summon using an artifact called the Quincy Cross, all of the Quincy possess one. This cross can also be a pentacle and its shape is important, the amount of lines within the cross denotes a Quincy's power level, with the most powerful receiving the pentacle.
The bow's power can be enhanced using the sanrei gloves. A Quincy needs to complete a week of intense training in order to be able to use these gloves. Upon forcibly taking off the gloves, the Quincy enters the final form and experiences an abrupt increase in power (along with a change in appearance), which absorbs energy from the surroundings at an alarming rate. In such a state the Quincy is able to fight with extreme strength, if only for a short time. Eventually this effect will wear off and the Quincy will completely lose all his powers, and will no longer be able to serve as a Quincy because the human body will seal itself from spiritual power, which is at that point already overloaded. It is possible to regain Quincy powers afterwards. Taking off the sanrei gloves is only used as a last resort.
The only way for a Quincy to regain their powers after using the final form is to first exhaust themselves mentally and physically, and then to be hit exactly 19 mm to right of their heart with a spirit arrow. After this a pentacle will form in the center of their chest and their powers will be restored. What new spiritual powers or limits they may possess afterward are unknown...