The projection of the Earth’s equator on the sphere of the sky. At the celestial equator, the stars seem to move in straight lines, although seen from different latitudes to the equator, those lines are actually arcs with a small curvature. Time and Date / Wikipedia.
Imaginary points in the sky where the Earth’s axis of rotation crosses the celestial sphere. At the geographic poles they will be just above in the sky (elevation of 90º). All objects at the celestial pole will appear not to rotate at all while the other objects, mostly stars, will appear to rotate around these points. Wikipedia.
The celestial sphere is an imaginary sphere that extends infinitely into space with the Earth at its center. The equator and the celestial poles coincide with the terrestrials. Time and Date
Circumpolar stars are stars that never get behind the horizon seen from specific latitudes. These stars are always visible by their proximity to the celestial poles, so visible stars in the northern hemisphere will not be visible in the southern hemisphere and vice versa. Wikipedia.
Comets are small celestial bodies, usually of dust and ice, orbiting the sun. Time and Date
A constellation is an area in the celestial sphere defined by the International Astronomical Union (IAU). Today there are officially 88 constellations. Time and Date