Active OS Fingerprinting Tools
These products are designed to guestimate remote operating systems and sometimes even the patch level that the operating system is running on. Active fingerprinting tools rely on stimulus-response, where the source will send certain packets (stimulus) to the target, the target's response can be analyzed to identify the operating system, Different Operating Systems respond to the source packets in different ways, hence their ability to fingerprint different remote hosts reliably.
See Also Passive fingerprinting tools. These perform a similar function but without stimulating the remote host, instead they rely on passing traffic.