CC, in reality terms, stands for Cubic Centimeters, which relates to the volume of the engine. As it is guessed, the higher the CCs, the faster the engine will go. As the speed of your Kart (or bike) increases, so does the difficulty of your fellow CPU Adversaries. You can view these difficulty levels in VS mode when setting the difficulty of the computers.
For every CC, all of the courses are available to be driven on. (You can see all of the course in depth, In this Article: Courses). Unlike the GCN Version, the difficulty of the track does not change depending the CC Level (Most notably, Luigi Circuit on 50CC and 100+ CCs)
50 CC: The easiest set of courses. Your kart travels at its lowest speed so you can take a longer time to make your decisions, and your reaction time doesn't need to be as fast. The CPU is also at its easiest level; it will never take short-cuts, they will always make mistakes, and they rarely hit you with items, including the Lightning Bolt and Leader Shell. ---When in a Grand Prix, this CC Level will only allow you to use karts---
100 CC: These courses are in medium difficulty. You lose a little bit of reaction time as your kart will move a little faster, and your computer opponents are set on "Medium" difficulty. Computers still won't take short-cuts, they will not make mistakes in their DRIVING (their aim for shooting items is still flawed), and they will start to hit you with every item they have. ---When in a Grand Prix, this CC Level will only allow you to use bikes---
150 CC: These courses are set at the second-to-highest difficulty. Your kart will move at its fastest speed, and your computer opponents are set on the "Hard" difficulty. Computers on this difficulty have been known to take shortcuts, and they will ruthlessly use items on you. The Computer's aim is near perfect (Green shells that hit you from bouncing off sides, as well as from behind when you least suspect it). And their driving skills are almost at the peak of performance (They will match times made by the beginner Staff ghosts). ---When in a Grand Prix, this CC Level will allow you to use both karts and bikes---
Mirror (150 CC): These courses are in the hardest difficulty. Everything is the same as the 150 CC Courses, but the courses are now mirrored. For every left turn, you now have to take a right. ---When in a Grand Prix, this CC Level will allow you to use both karts and bikes---200 CC: 200 CC is available for Mario Kart 8. It has even bigger speed and even sharper turns. However, it will not be added to Mario Kart Wii.