Spyware has been in the news recently with stories like the Apple security vulnerability that allowed devices to be infected without the owner knowing it and a former editor of The New York Observer being charged with a felony for unlawfully spying on his spouse with spyware. Spyware is a sub-category of malware thats aimed at surveilling the behavior of human targets using a given device where the spyware is running. This surveillance could include but is not limited to logging keystrokes capturing what websites you are visiting looking at your locally stored filespasswords and capturing audio or video within proximity to the device.
Spyware much like any other malware doesnt just appear on a device. It often needs to first be installed or initiated. Depending on what type of device this could manifest in a variety of ways but here are a few specific examples.
With all the above examples the bottom line is that there could be software running with a surveillance intent on your device. Once installed its often difficult for a lay person to have 100 confidence that their device can be trusted again but for many the hard part is first detecting that surveillance software is running on your device.