An interchangeable lens is attached to an SLR or mirrorless camera via a locking mechanism called a "mount." Once it's mounted, the lens provides you with a specific "focal length." (Technically, that focal length is the distance between the lens' glass elements and the camera's sensor, which captures the image). The greater a lens' focal length, the closer it brings the subject you are shooting. A given lens is usually designed to work with just a single brand of camera. For example, a Nikon lens will fit a Nikon DSLR or mirrorless model, but not a Canon. See the Brands section for more information.
A lens is like a tube filled with glass lenses that funnel light onto a digital sensor. The physical size of the lens, and the amount and shape of the glass will affect its focal length. A standard or "normal" lens, with a fixed focal length of 50 mm, has a horizontal angle of view that is about the same as what the human eye perceives. A longer lens, such as a telephoto, will bring you closer to your subject while a shorter, rounder, wide-angle lens will give you a broader perspective.
Some lenses, called zooms, provide a range of focal lengths that you can control as you shoot. Some of the most typical zoom lenses used with consumer SLRs or mirrorless cameras range from 18 to 55 mm to 24 to 85 mm. For shooting subjects at greater distances, there are telephoto zoom lenses, with ranges such as 70 to 200 mm. For shooting landscapes or other wide subjects, there are wide-angle zooms, with a range such as 12 to 24 mm. For the greatest versatility, there are ultra zooms, with ranges as great as 28 to 135 mm or 18 to 200 mm.
Because the image sensors on digital cameras are usually smaller than a frame of traditional 35-mm film, when a lens is used with a digital SLR or mirrorless model, you need to account for that difference by figuring its effective focal length. For example, a 50-mm lens on a Canon Digital Rebel would have an effective (or "equivalent") focal length of an 80-mm lens when taking into account the camera's 1.6x magnification. Such magnification factors vary from one brand of camera to another, and sometimes even among models within the same brand.
Some higher-end, digital, interchangeable-lens cameras use so-called "full-frame" sensors, which are about equal in size to a frame of 35-mm film. With such cameras, there is no lens magnification, or equivalent focal length. In short, a 50-mm lens remains a 50-mm lens on a full-frame sensor SLR such as the Canon EOS 6D or Nikon D610.