A CNC router machine is probably the most useful tool a hobbyist can own, but the price for a CNC machine on the market is way more than the average hobbyist is willing to spend. You can build your own CNC with very basic tools, little knowledge of machinery, mechanics, or electronics, but be warned, these machines are inherently dangerous, so wear the proper protection and use common sense. At the very least, read the instructions and precautions on every tool you use.
Now, a CNC stands for Computer Numerical Control, but that is gibberish. It does hint you to the fact that something is controlled, and probably by a computer. It's really a mechanism that interfaces to a computer to control its movements, similar to a robot, but this machine has a specific function. It cuts or shapes things with this control. In essence, a computer precisely controls a cutting tool, like a router for instance, to cut materials such as wood, plastic or metals into shapes only bounded by your imagination.
This video tutorial will show you how to make the gantry sides for a CNC router. The gantry is the part of the CNC Router that moves along the x-axis. That's what makes this machine a gantry style machine. The gantry allows the router to essentially float over the cutting surface. The gantry sides consist of two boards approximately 18" x 8". The linear slide bearings are longer than the other linear slide bearings on the y and z axes so the weight of the gantry can be supported. The gantry needs to carry the z-axis, router, and the y-axis with all support pieces. In addition, the force of the motors against the force that the bit and cutting imposes on the machine must be handled by the entire gantry.