Spyware is a category of software for computers. Spyware is malware that collects some data, usually without the computer users' knowledge. Very often, this data is then sent over the internet to someone else. Very often, this is used for marketing. Spyware can also be used to steal data from computers. One kind is a keylogger which can see whatever you are typing. Keyloggers can steal important information like passwords that you type.