After prototyping a project using tools like a solderless breadboard, small-run or one-off PCBs are a good choice when you want a more structurally stable version of your circuit that is destined for mass production. Perhaps you would like to be able to install it into a larger project prototype. Once all the what-ifs and tests are successful with these hand-made PCBs, higher resolution industrial processes can be used to fabricate more robust PCBs.
Thanks to members, mirage335 and Dan Barlow, we now have a standardized method to fabricate DIY PCBs with traces (conducting areas) and spacing (non-conductive areas) of 6 mil (0.15mm).
For projects where the 10 mil tolerances made possible by our CNC mill are not enough, photolithography is a good alternative. The process uses light-sensitive coatings to mask conductive surfaces before etching. The result is a a lovely printed circuit where before you had a jumble of wires. The process, workflow, and references are described by mirage335 on our wiki.