Speed, always refers to the spindle and the workpiece. When it is stated in revolutions per minute(rpm) it tells their rotating speed. But the important figure for a particular turning operation is the surface speed, or the speed at which the workpeece material is moving past the cutting tool. It is simply the product of the rotating speed times the circumference (in feet) of the workpiece before the cut is started. It is expressed in surface feet per minute (sfpm), and it refers only to the workpiece. Every different diameter on a workpiece will have a different cutting speed, even though the rotating speed remains the same.
Feed, always refers to the cutting tool, and it is the rate at which the tool advances along its cutting path. On most power-fed lathes, the feed rate is directly related to the spindle speed and is expressed in inches (of tool advance) per revolution ( of the spindle), or ipr. The figure, by the way, is usually much less than an inch and is shown as decimal amount.
Depth of Cut, is practically self explanatory. It is the thickness of the layer being removed from the workpiece or the distance from the uncut surface of the work to the cut surface, expressed in inches. It is important to note, though, that the diameter of the workpiece is reduced by two times the depth of cut because this layer is being removed from both sides of the work.
source : http://www.mfg.mtu.edu/marc/primers/index.html
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