Spyware may refer to any of the following:
1. Spyware or snoopware is a term used to describe a software program that is intentionally installed on a computer. Spyware is used to monitor what other users of the same computer are doing.
2. Spyware is a term used to describe a program designed to gather information about a user's activity secretly. Spyware programs are often used to track users' habits to target them with advertisements better. Spyware is usually installed on a user's machine without their knowledge when a link is followed (intentionally or unintentionally) which redirects the user to a malicious website.
Protecting a computer from spyware
A computer can be infected by spyware by visiting an infected website, downloading infected software, or installing infected software. Most antivirus programs can clean spyware, but it's also a good idea to run an anti-malware or malware cleaner such as Malwarebytes.
How do I know if I have spyware?
Unless something is noticeably different, the only method of detecting spyware on a computer, phone, or tablet would be to have an anti-spyware or anti-malware detection tool.