Aperture vs. Magnification
All telescopes collect and focus light. The most important feature of any telescope is its light gathering capability, or aperture (the diameter of the main lens or mirror, usually expressed in inches or millimeters) that determines how much you will be able to see. The more light gathered, the more you'll see. For example, a 6" mirror/lens is 2-1/2 times the diameter of a 60mm mirror/lens, yet it gathers 5-1/2 times more light. The brightness-boost delivered by the bigger, high-quality optics seems to yield dramatically finer detail, simply because previously invisible structure gets promoted to the visible—or in other words, the resolving power is increased.
Large apertures (more than 8") will yield sharper images only when that area's stable atmospheric conditions (primarily air turbulence) promotes excellent "seeing". Most of the time in most of the world, seeing is moderate to poor. Magnification is not the most important consideration when choosing a telescope. Telescopes advertised on the basis of high magnification (400X to 575X power, or more) are a sure sign of inferior quality. Excessive magnification blurs the already blurry image, mostly by amplifying the effects of air turbulence. In practice, 300X is about the maximum. The brightest and sharpest images are obtained from much lower powers, on the order of 30X to 50X. The telescope's power or magnification is adjusted by changing eyepieces. How many eyepieces should you have? Three is ideal—one low, one medium and one high magnification. The lower its focal length, the greater the magnification. For a larger image, use an eyepiece with a larger number (i.e., 20mm). For close-ups, use a smaller number (i.e., 4mm). You can buy other eyepieces to increase the versatility of your telescope, or you may want to purchase a Barlow 2X lens that is inserted into the focusing tube before the eyepiece which doubles the magnification—a good way of getting double the performance out of each eyepiece! Apart from making the image of the subject larger or smaller, magnification has a bearing on the area of sky (termed the field of view) you will see—higher magnifications have smaller fields of view, which can make finding objects that much more difficult.
Generally speaking, lower powers of 30 - 50X are advisable for observing star clusters, galaxies and nebulae since they are often spread over a wide area of sky. Medium powers of 80 - 100X are convenient for studying the craters and valleys of the Moon's surface, seeing the rings of Saturn, or Jupiter and its four principal moons.
Higher powers of 150 - 200X will permit you to scrutinize mountain peaks and fine lunar detail, the surface features of Mars or to separate close double stars.
For example, if your telescope has the focal length of 1000mm and you are using a 20mm eyepiece you would be viewing at 50 power (1000/20=50).
Focal lengths are expressed in millimeters. You will find the focal length of the eyepiece printed on the eyepiece itself and the focal length of the telescope printed on the telescope assembly tube itself or in the manual.