Any basic astronomy textbook has a brief description of coordinate systems. This system is based on the terrestrial globe. The celestial equator and poles are projections of the terrestrial equator and poles.
Declination is analogous to latitude.
Right Ascension is measured in units of time. This is the point where the ascending node of the apparent Solar orbit crosses the celestial equator. The ecliptic coordinate system is based on the apparent Solar orbit, and is the natural system for Solar System studies. The poles are projections of the Earth's orbital poles. The origin is the same as that of the celestial system.