Time to dispel jargon myths with a bit of a telescope glossary. The larger a telescope's aperture (the size of its main lens or primary mirror) the more light is collected and more fine detail is revealed. For example, a 200 mm aperture collects four times more light than a 100 mm telescope. Under ideal conditions, a 100 mm telescope reveals stars down to magnitude +11.8, while a 200 mm telescope will show stars down to magnitude +13.3. A 100 mm telescope will 'split' a double star separated by 1.5 arcseconds and resolves a three-kilometer lunar crater; however, a 200 mm telescope resolves a crater just 1.5 km across and a double star separated by just 0.6 arcseconds. Resolution is limited by the telescope's optical quality and the steadiness of the Earth's atmosphere.
Telescopic magnification depends on the telescope's focal length (the distance between the objective lens/primary mirror and the point of focus of the light it collects) and the focal length of the eyepiece used. Magnification is calculated by dividing the telescope's focal length by the eyepiece's focal length. For example, a 100 mm telescope with a focal length of f/8 (eight times the telescope's aperture) has a focal length of 800 mm; used in conjunction with an eyepiece of 10 mm it will deliver a magnification of 80 times (800 divided by 10).
With any telescope, the range of useful magnification depends on a telescope's aperture and focal length, combined with the focal length of the eyepiece. Too low a magnification (taking in as wide an area as possible) will actually waste light since the 'exit pupil' of the eyepiece will be larger than the diameter of the pupil of your dark-adapted eye. On average, the adult pupil will dilate to around 7 mm in dark conditions. Therefore the exit pupil delivered by an eyepiece ideally needs to be 7 mm or smaller so that all the light gathered by the telescope — especially when attempting to see faint objects — can be taken in. Exit pupil can be calculated by dividing the telescope's aperture by the magnification delivered by any particular eyepiece.