A keylogger is a spy tool used to capture your keystrokes. The most common keyloggers are software based. There's also hardware keyloggers. A keylogger is a double-edged sword, meaning it can be used for good and bad. It might be used by parents or guardians to monitor their children's internet activity or be used by criminals to steal your passwords, social security numbers, and credit card numbers. Keyloggers might be attached to trojan/viruses. Keyloggers hide and are very hard to detect by a novice. They don't show up in processes. Keyloggers might hook to your keyboard which is the most common way or inject its self deep into the kernel of your operating system which makes it very difficult to detect and remove.
I had all of these charges on my credit card I didn't make. Later I found out that some one installed a keylogger in my computer without me knowing and captured my keystrokes and stole my credit card number.