A huge step up from the classic compact is being able to change lenses. This you can do, not only on DSLRs, but on mirrorless cameras such as the Olympus PEN or the Nikon 1, amongst others. This presents lots of advantages, the main one being versatility. There is no one lens to rule them all, and even if there were, it would set you back a small fortune. So people try to get lenses to cover all bases – enter the focal length.
On compact systems, we’re used to knowing the amount to zoom by, e.g. 16x or 32x optical zoom. With more advanced cameras, you don’t really get the information presented to you in this form, rather, as millimetres of focal length, for example, 18-55mm or 70mm. This tells you the minimum distance away a subject has to be to be in focus, and the zoom capability. How? The zoom can be worked out by dividing the larger number by the smaller number. In the 18-55mm kit lens, you’d therefore get roughly 3x zoom (55/18). On the 70mm lens, you would get no zoom (70/70=1). These are called prime lenses and sacrifice zoom for larger apertures which means faster shutter speeds and shallower depths of field.
Wide angle lenses typically have focal lengths of 10mm, telephoto lenses – 200mm, 300mm or more. Be wary of high zoom lenses though, they get difficult to use handheld because as you magnify the image, you magnify each microscopic twitch in your hands too.