This is my attempt at an explanation: A router is a wood working tool that many people have used in place of a spindle on their CNC machine. Commonly used routers are made by Bosch, DeWalt, and Makita. I personally use a Makita. Each one needs a slightly different sized mount. They are generally cheaper than a spindle. However, they really weren't designed to run non-stop for hours on end, and will occasionally need to have their brushes replaced. Spindles are made for the CNC application. Most people get them from China and many come with a speed controller that your CNC hardware can control. They are either air cooled or water cooled. Spindles are also quieter when they run, but for me that did not matter because with my vacuum system and compressor for blowing away chips both running, as well as the noise of the endmill cutting the material, I do not hear my router.
As Craig said, the spindle runs much quieter than a Router. It also runs in conjunction with a VFD, which gives a far more accurate and visual speed control. Their availability is predominantly from China, which makes them cheaper nowadays, but there is, of course, the usual time delay in getting one.
Router: A router runs loud this has to do with the type of frequency's involved in turning the motor. Spindle:A Spindle in Quiet in comparison because it drives with a High frequency. Routers : Can plug directly into the wall. Spindle: requires a 3phase inverter/driver. Routers: have higher run out Spindles: have a or should have a very low run out. Routers: are more prone to bogging down wile putting heavy loads causing finish problems Spindles: can take a lot more load without bogging. Routers: are cheap Spindles:are not Routers:are easy Spindles:are not Routers: are air cooled Spindles : may be water cooled and you need pumps tubes water etc.. There are more differences but this should give you a good idea
All of what Gooshpoo said is Spot on with his run down of the comparison between Routers & Spindles But Also the typical 1.5KW & 2.2KW Chinese spindles tend to be much more heavy in weight ( 3X +) compared to a standard woodworking router & You need to take this in to account when adding one to your CNC Machine. Questions you need to ask yourself are : Is your Z axis Up to the task of moving this large mass Up & Down , Also If your Using a moving gantry machine you need to be sure its up to the task & Won't Flex. One Other Extra Good Point about Spindles over Standard Woodworking Routers is you can use various sizes of end mills Example - A Typical Chinese Spindle that comes with a ER20 Collet set can use 1.0mm to 13.0mm end mills and every size in-between as standard & You can also get high precision collets with a Run-Out <5 Microns. - You Would not need this type of accuracy if your just cutting wood ! - All Depends on your machine & what you Intend to use it for. Hope this helps you Decide
