Footprinting is used in information technology to refer to efforts to find out about computer systems and their networks, or footprints. Although footprinting can be done for legitimate purposes, the term is often linked to hacking and cyber attacks.
Passive footprinting requires no direct contact with the organisation being researched. Active footprinting may mean some direct contact or interaction with the organisation and usually includes social engineering, entering the premises, and taking photos or shooting videos.
Reconnaissance is a more military term referring to system information collection for hacking purposes or system penetration testing. In active reconnaissance, a hacker uses system information to gain unauthorized access to protected digital or electronic materials, and may go around routers or even firewalls to get it. Active reconnaissance is also used by systems analysts and programmers to test the security of networks and systems and scan for potential vulnerabilities.
OS fingerprinting is the process of determining the operating system used by a host on a network. Active fingerprinting is the process of transmitting packets to a remote host and analysing corresponding replies. Passive fingerprinting is the process of analysing packets from a host on a network. In this case, fingerprinter acts as a sniffer and doesn't put any traffic on a network.
TCP/IP stack fingerprinting is the passive collection of configuration attributes from a remote device during standard layer 4 network communications. The combination of parameters may then be used to infer the remote machine's operating system, or incorporated into a device fingerprint.
A fingerprint in hash cryptography is a short key that helps to identify a longer public key. Fingerprints are used for key authentication and other elements of cryptography security, providing efficiency with smaller data sets.