To help locate these stars, the celestial sphere has a grid system. Just as we have longitude and latitude on the earth to locate a city, we have a similar system in the sky to locate a star, or an object.
The blue lines on this figure demonstrate the grid system used. The heavier blue line is the celestial equator, placed directly over the earth's equator.
Square to the equator are the poles. The North Pole is visible on this celestial sphere (it is the point where all the blue lines intersect), as the sphere is set for London, which is located at a northern latitude. You can also see the North Pole star Polaris, on the North Pole.
The blue lines that run from the North Pole down to the South Pole (out of sight in the figure) are known as lines of Right Ascension, and are equivalent to lines of Longitude on the earth. The blue lines that are the smaller circles stepped towards the poles are known as Declination, and are equivalent to Latitude on the earth.
Thus any star can be located on the sphere by measurements of Right Ascension and Declination, just as any city on the surface of the earth can be located by measurements of Longitude and Latitude.