The precession of the equinoxes is due to a wobble in the earth's axis with relation to the plane of the ecliptic (or, with relation to the fixed stars.) At present our north pole points at the star Polaris, the "pole star". But 13000 years hence it will be pointing toward Vega. If you are familiar with the sky, step outside and see how far apart Polaris and Vega are (about 23-1/2 degrees).
Due to this wobble, the point in the distant sky (among the fixed stars) where the vernal (and autumnal) equinox occurs slides forward in the zodiac in a cyclic motion that spans about 26,000 years. The point at which the vernal equinox occurs is measured each year and is precisely known. There is some minor disagreement as to exactly where the vernal equinox occurred at some point in the past (the basis of the sidereal zodiac, called an ayanamsa). Currently the difference between the two zodiacs is about (depending on the ayanamsa used) 24d03m (using Lahiri ayanamsa, the official ayanamsa of India). This precessional motion thus amounts to one degree in about 72 years (360x72=25920). This is the reason behind the use of two zodiacs, the sidereal (based on a fixed position among the fixed stars -- a fiduciary) and the tropical (based on the annual occurrence of the vernal equinox, as measured from year to year.)
The world's astronomers and most western astrologers use the tropical zodiac, while the Vedic astrologers and a minority among western astrologers employ the sidereal zodiac.
The cause of the precession is complex, but is due primarily to gravitational influences from Moon and Sun on Earth's equatorial bulge.
Hipparchus is credited with the discovery of the precession (he used Spica, which is today the fiduciary of some siderealists, and found that it was about 2 degrees from where previous observations placed it with regard to the equinox.) [There is an article about him in wikipedia. There is an error in the article. It credits Hipparchus with the invention of the armillary sphere, which was in fact invented by Eratosthenes about a century and a half before Hipparchus.]
You may find this article helpful:
http://www.astro.com/astrology/in_praezession_e.htm
I don't know what you mean by the Placidus orb system. Are you referring to house systems, or something else?
