A Wi-Fi sniffer is a device or software tool that monitors and captures data transmitted over wireless networks. It listens to Wi-Fi signals in the area, allowing users to analyze network traffic, identify devices, and detect vulnerabilities. While Wi-Fi sniffers can be used by network administrators for troubleshooting and securing networks, they can also be misused by cybercriminals to intercept sensitive information, such as passwords or personal data, from unsecured networks. Proper security measures, like strong encryption and secure passwords, help protect against unauthorized Wi-Fi sniffing.