Spyware is a type of malicious software that secretly installs itself on computers or mobile devices without the user's knowledge or consent. Its main purpose is to monitor user activity, collect sensitive information such as passwords, credit card numbers, or browsing habits, and transmit this data to third parties. Spyware can slow down devices, compromise privacy, and lead to identity theft or financial loss. It often spreads through infected downloads, email attachments, or malicious websites, making it important to use security software and practice safe browsing habits to prevent infection.