Ctrl-C, also sometimes written with a plus instead of a minus like Ctrl+C or Control+C, has two purposes depending on the context in which it's used.
One is as the abort command used in many command line interfaces, including the Command Prompt in Windows. The Ctrl-C keyboard shortcut is also used to copy something to the clipboard for the purpose of pasting it somewhere else.
Either way, the Ctrl+C shortcut is executed by holding down the Ctrl key and simultaneously pressing the C key once.
Command+C is the Mac OS X equivalent.
How to Use the Ctrl+C Shortcut
Like I mentioned above, Ctrl+C behaves differently depending on the context. In most command line interfaces, Ctrl-C is understood as a signal instead of text input, in this case used to stop the currently running task and return control back to you.
For example, if you executed the format command but at the initial warning decided against completing it, you could execute Ctrl-C to cancel the format before it started and return to the prompt.
Another example at the Command Prompt would be if you were to do execute a dir command to list the directories of the C: drive. So, say you open a Command Prompt at the root of the C: drive and execute the dir /s command - all the files and folders on the entire hard drive will be listed out. Assuming you weren't using the more command with it, that would take a while to display. Executing Ctrl-C, however, would immediately interrupt the output and return you to the prompt.
If you're running some sort of command line script that seems to be in a loop when you know it should be finished running, you can stop it in its tracks by interrupting it with the Ctrl+C keyboard shortcut.
The other use for Control+C is to copy something, like a group of files on your desktop, a sentence or single character in a line of text, a picture from a website, etc.
It's the same function as right-clicking something and choosing copy. This command is recognized all throughout Windows and pretty much every Windows application you might be using.
The Ctrl+C shortcut is then usually followed by Ctrl+V to paste the most recently copied information from the clipboard to wherever the cursor sits. Just like copying through the right-click context menu, this paste command is accessible in that way too.
Tip: Ctrl-X is used to copy text to the clipboard and simultaneously remove the selected text from its source, an act called cutting text.
More Information on Ctrl+C
Ctrl+C won't always interrupt an application's processes. It's entirely up to the specific program as to what the key combination will do, which means it's possible that some programs with a command line interface won't respond in the same way as described above.
This is also true for software with a graphical user interface. While web browsers and other programs like image editors make use of Ctrl+C for copying text and images, the occasional application will not accept the combination as a command.
Software like SharpKeys can be used to turn keyboard keys off or swap one for another. If your C key isn't working as it's described here, it's possible that you've used this program or one like it in the past, but have since forgotten that you've made those changes to the Windows Registry.