Which of the following best describes spyware?

Spyware is defined as a type of malicious software designed specifically to infiltrate a user's computer without their knowledge and collect sensitive data. This information can include personal details, browsing habits, and login credentials, which can then be used for identity theft or sold to third parties. Its primary goal is stealthy data collection, often leading to privacy violations and potential financial harm to the user.

The other options do not accurately capture the essence of spyware. Enhancing system performance, providing legitimate system information, or aiding in file recovery are functions associated with different types of software that do not involve covert data collection or malicious intent. Thus, the focus on data collection in the correct answer distinctly characterizes spyware's functionality.