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 by to monitor what other users of the same computer are doing.
2. A term used to describe a software program that has been designed to secretly gather information about a user's activity. Spyware programs are often used to track users' habits to better target them with advertisements. Spyware is usually installed onto 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 a spyware or anti-malware detection tool.