The word telescope originates from the Greek language (τηλεσκόπιο), and it means literally far-seeing. It is an instrument that is intended to gather information, usually in the form of light or other energy, about a distant location that can not be perceived directly by the unaided eye. Telescopes let you magnify faint stars, and see finer detail. This allows you to, for example, distinguish one star as two stars that are very close (such as Mizar A and B).
Telescopes are used by astronomers to gather visual and other information the human eye alone cannot see. They do so by means of a large collector surface that then directs the incoming energy photons into a smaller focusing lens. This large surface also allows the resolution of fine details that are too close together for the human eye to discern. It was through his usage of the telescope that Galileo discovered that the celestial spheres weren't perfect, as had been thought. He could see sunspots, and the lunar "seas".
Today, observational astronomy covers the entire range of the electromagnetic spectrum. Telescopes detecting other fundamental particles are also very common. Nonetheless, we shall start with our most familiar part in the field, the optical region in the spectrum.
Some difficulties with telescopes are chromatic aberration (different colors focused at different points) and spherical aberration (light isn't focused at a single point). Spherical aberration can occur when the reflecting mirror in a telescope isn't a true parabola.
Adaptive optics are used to compensate for atmospheric distortion which is caused by small changes in the refractive index of the atmosphere, which is responsible for the "twinkling" effect.
The Light Gathering Power of a telescope is measured by the area of the lens divided by the area of the eye.
Large telescopes are housed inside observatories. One of the most famous is the Lick Observatory on Mt. Hamilton, named after James Lick, a streetcar baron, who made donations to have the observatory built. When he died, he was buried beneath it.
Light pollution makes stargazing difficult. This is caused by fixtures (such as streetlamps) that allow light to go up into the atmosphere where it is reflected and creates a "fog" of light that makes viewing very dim distant objects difficult.
Our sense of sight is based upon particles of electro-magnetic energy that are emitted by objects and then collected by our eyes. These particles are vibrating quanta of energy that are known as photons. Every photon oscillates at a particular frequency, and the range of all possible frequencies is called the spectrum.
Human eyes are attuned to a particular frequency range of photons that lie within the visual portion of the spectrum. We can not directly view photons that vibrate at frequencies above or below this range, although measuring instruments have been constructed to measure these parts of the spectrum.
All the information we have about the planets in our solar system, and the stars and galaxies comes to us from photons that have traveled that great distance. It is almost exclusively by means of these photons that astronomers have been able to assemble our current knowledge of the universe beyond the Earth.
(Some people are working on a device to try to detect "gravitons" that have traveled that great distance.)