Nmap (Network Mapper) is one of the most simple and efficient tool that can be used for Penetration testing. It is an free and open source tool that is used for network discovery and auditing available in Kali Linux, Windows and Mac OS. This tools helps system and network administration to perform various task like network monitoring, discover port and host status, gather information, Network inventory etc. Nmap uses raw IP or domains to determine the hosts available on the corresponding, its port status, various services available under each ports, what operating systems they are running, presence of filtering techniques over the network and many more.
Basic Commands
In this section, basic commands that can be used with Nmap is listed below:

Reference : https://tools.kali.org/
