Exposure compensation: In a digital camera, this function allows you to override the exposure recommendation given by the built-in metering system. For example, if a scene you're capturing is repeatedly exposed as dark, you might use exposure compensation to lighten up the image.
Focal-length multiplier: This is one of the most misunderstood terms used with digital SLRs (it's also referred to as the magnification factor). Most digital SLR cameras use an image sensor that's smaller than a 35mm film frame. Because of this, the image sensor is seeing only part of the image presented by the focal length as compared to that same focal length on a full-frame 35mm SLR. The most convenient way of describing how the smaller image sensor affects the performance of the lens is to say something like "a 100mm lens will act like a 160mm." What's actually happening is that the field of view of the 100mm seen by the sensor is changing, not the focal length.
Histogram: This is a graph displayed on the LCD of many cameras. It's a diagram of the dark to light range of the captured image. Typically, the left side of the histogram is the dark part of the image and the right is the bright. If all of the data on the graph is on the left of the scale and it's clipped (cut off sharply) at the left edge, the image is underexposed; if the data is on the right side and also clipped, it's overexposed. You'll also see a histogram pop up in many image-editing programs so that you have a guide while making image level adjustments for setting black and white points.
IS or VR: Image Stabilization (Canon) or Vibration Reduction (Nikon) is technology in the optical system that compensates for tiny camera movements. (A stable camera is key to getting sharp photos.) This technology involves sensors in the lens that detect movement and a mechanism for making slight adjustments to a movable lens element. Some systems make those corrections by moving the image sensor (called AS for anti-shake). In some video cameras, the corrections are made on a frame-by-frame basis by shifting the active pixels on the sensor to compensate.
Metering systems: These include Evaluative, Honeycomb and Matrix. They're how manufacturers have set up their cameras for determining the best exposure for a scene. They all sample light readings in various parts of the image frame (usually a minimum of eight, but they can go up to several times that), then compare them to determine a possible exposure. Some cameras take this process a step further by examining the data from the light meter and comparing it to a database of image scenes. These systems can be successful in achieving good exposures in difficult light situations.