A case mod is any modification performed on a stock computer case. Mods can be functional or purely decorative, but they're all created in the name of creating a custom computer.
One of the most basic decorative mods involves adding see-through panels to case sides. You can use a rotary cutting tool to open a space in the side and then insert a plexiglass or acrylic panel. The main idea is to show off your hardware, but once everyone can see the inside of the computer, then it's also possible to add cold cathode or LED lights. Case lights have no functional use, but they can look quite impressive and further highlight your hardware.
A popular functional mod is adding extra fans to a case. Once again, you can use a rotary cutting tool to create additional holes in the case and then insert fans of your choice. Extra fans create cooler temperatures within the case, leading to better performance by allowing for the hardware to run at higher speeds.
Computer owners can also highlight fans through the use of replacement grills. Why have plain grills when a biohazard sign or lightning bolt can grace your computer?
Another functional mod is adding an after-market cooling system to a computer case. Like additional fans, these cooling systems are added to allow the computer to run at peak performance. They typically use water flowing through flexible tubes, along with pumps, reservoirs and radiators, to lower the temperature of the components they touch. You can also add a chemical to the cooling system water that will be UV reactive to case lights, adding further glow to your case.
The best mods combine multiple modifications to create a uniform look or theme. Hardcore gamers create mods based on their favorite games, and others just go for design elements like flames. No matter what the end result is, creating a custom case mod means that you have a unique computer, designed to your tastes and needs.