The purpose of a telescope is to:
1. Gather light (make things appear brighter than they do with the naked eye)
2. Magnify the image (make things appear larger than they do with the naked eye)
Gathering light: Refractor telescopes gather light based on the size of the main lens. This main lens is called the "objective." The larger the objective the more light it gathers. Since objects in space are extremely far away and very dim, the more light you gather, the more you will see. Our smallest telescope is the SV60EDS. It gathers 73 times the amount of light the naked eye does. Our largest telescope is the SV160T. It gathers 522 times the amount of light our naked eye does.
Magnifying the image: Binoculars often operate at about 7 - 12X meaning objects appear 7 - 12 times larger than they do to the naked eye. Larger telescopes can magnify more than 200X, making small objects like planets appear large enough so we can see details on the surface. These magnification powers also allow us to split close double stars and make out small regions of faint nebulosity (gaseous clouds in space).
While a telescope gathers light based on the size of the objective, it magnifies based on the eyepiece used. Different eyepieces provide different magnification powers. This allows the user to select a variety of magnification powers. Different objects require different magnification powers. A very large extended object like the Andromeda Galaxy requires a very low power which affords the widest possible view. Star clusters are best seen at medium magnifications. Planets and double stars are best viewed at higher magnification powers.
The better the optics the clearer things will appear. Two problems that can result in a poor night of viewing are:
1. Poorly made, mass produced optics, that can magnify to the same power as a high end Apo refractor, but make the object appear blurry and leave small details invisible (the object is large but you cannot see it clearly), an effect we call "empty magnification."
2. Air turbulence in the upper atmosphere. Even if the air is still all around you, it may be turbulent at higher altitudes. This will blur details limiting the amount of magnification power you can use. Twinkling stars is an indication of air turbulence. On such nights it is best to lower the magnification power to get the sharpest view. Turbulent nights are best for viewing objects like star clusters and nebulae. Steady nights allow one to boost power and get a closer look at lunar craters, close double stars and fine details on the nearer planets.