Nmap (Network Mapper) is a security scanner originally written by Gordon Lyon (also known by his pseudonym Fyodor Vaskovich) used to discover hosts and services on a computer network, thus creating a "map" of the network. To accomplish its goal, Nmap sends specially crafted packets to the target host and then analyzes the responses. Unlike many simple port scanners that just send packets at some predefined constant rate, nmap accounts for the network conditions (latency fluctuations, network congestion, the target interference with the scan) during the run. Also, owing to the large and active user community providing feedback on its features and contributing back, nmap has succeeded to extend its discovery capabilities beyond basic host being up/down or port being open/closed to being able to determine operating system of the target, names and versions of the listening services, estimate uptime, the type of device, presence of the firewall.
Nmap runs on Linux, Microsoft Windows, Solaris,HP-UX and BSD variants (including Mac OS X), and also on AmigaOS and SGI IRIX. Linux is the most popular nmap platform with Windows following it closely.
Nmap is a software that search the host for the open ports and the services available that is the first step in hacking a website or computer.