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People at the RepRap 3d-printing project often mention CNC routers or CNC mills.
Both kinds of machines almost always have a motorized spindle with stepper motors to move the spindle in the X, Y, and Z directions.
What is the difference between a CNC router versus a CNC mill?
(Is there a better place for this sort of question -- perhaps the Woodworking Stack Exchange?)
TL;DR CNC routers move the tool while CNC mills move the workpiece, but this is a matter of common usage and not based on hard definitions.
A router (in the woodworking sense) is just a rotary tool with a cutting bit. Woodworkers consider them a very universal tool, because they can be mounted into shop-made fixtures to create a wide range of other tools.
A "mill" typically defines a machine that can feed a workpiece to a rotary cutting bit along several axes. There are many variations on this concept (e.g. vertical mills, horizontal mills, spiral mills, etc) depending on which axes are used.
Technically speaking, either the workpiece or the cutting tool can move in a milling machine. But in common usage, when people say "milling machine" they mean "bridgeport-style milling machine" -- a vertical mill with its cutting tool in a fixed position and an X/Y/Z axis table that can move the workpiece. (A mill made from a router is just a "gantry-style" milling machine.)
Not a big fan of using head motions or type of spindle to define the difference, as there are lots of exceptions to each. Many 5-axis mills move the head in various ways beyond just Z. Datron makes CNC mills (excellent ones, BTW) that look like these CNC "Routers" but they call them "Mills".
Some refer to "Gantry Style" mills as routers. The "Gantry" is that travelling bridge with the head on it. But, Some gantries still move the table, often in just one axis, usually Y.
There are CNC "Routers" that have very expensive high speed spindles with toolchange capability. They're not just lashing a woodworking handheld router onto the CNC.
In the end, "Router" seems most often to refer to a gantry-style machine that is most often used to cut wood and soft materials like plastic or foam board. "Mill" seems to most often refer to machines that are usually not (but can be) gantry-style that are most often used to cut metal and harder materials.
Personally, I prefer to use the terms interchangeably and realize that you can use one to do the other's job in many cases.
On a typical CNC router, the work is stationary and the spindle-head moves in x/y/z coordinates.
On a typical CNC mill, the work moves in x/y coordinates (ie, is attached to an x/y table) and the spindle-head moves in the z axis only.
An example of each is shown below (photos from pdjinc.com and hylands.net):
Generally, the term comes from how the machine derives its cutting power.
A mill generally cuts slow RPM, from 3,000 to 5,000 with a lot or torque.
A router will usually run 10 times that with 30,000 to 50,000 RPM with much less torque.
The axis movements as mentioned by jwpat7 also apply.
CNC machining is popular in the manufacturing sector with the benefits it offers. You can see different types of CNC machines get developed, each having its advantages. The two most popular ones are the CNC router and CNC mill. They seem similar but are different. Here are the differences:
CNC router
CNC routers are designed to operate at high speeds compared to CNC mills, using the rotational speeds as the driving force to make the cuts. While routers have three axes, there are variations with 4 or 6 axes ideal for complex projects.
CNC router highlights:
CNC mill
CNC mills can perform nearly all the functions of the CNC router, but there are some differences. CNC routers are for softer materials, and mills get used to cut hard materials. Instead of a stationary table, the CNC mills move in X and Y configuration, with the spindle moving on the linear axis above the workpiece.
CNC mill highlights
