The most common bit to do pcb routing is a V bit, sown in the image below. The angle of the V is a large determining factor in how fine of traces you can cut verses how flat the pcb material needs to be. Common bit angles include 90, 60, and 45 degrees.

In the picture below are illustrations of the three different cutter angles mentioned. The black outine is the 90 degree cutter, the blue is the 60 degree cutter and the red 45 degrees. For those who want to cut fine trace pcb's and have difficulty holding their work flat, the 45 degree cutter is the most forgiving. 45 degree bits are more fragile and dull quicker than a either a 60 or 90 degree bit.

My favorite bit is the 45 degree bit from Drewtronics. I've also been happy with the quality of cuts with the mschanical etching bits from Think N Tinker. In my opinion, the Drewtronics bits last longer and are more durable. Below is a quick reference on how cutter angles of Vbits affect cut width. The yellow grid is 1mil increments (.001 inch), four cutter angles are shown 90, 60, 45 and 30 degree bits. For example a 45 degree bit (Red) at 2 mils is a little over 2 mils cutting width, at 4 mils depth it is closer to 5 mils. This assumes there is no vibration and the spindle has no out of round characteristics, which is not the norm. So actual cut width is normally higher.