The host file is an important file that resides in operating systems that have a networking capability. The host file is a simple text file - sometimes referred to as a plain or "vanilla" text file - that contains, as the name would suggest, the name of hosts found on a computer network. The name of a host, or hostname, is a word given to identify a device or computer that is connected to a computer network. Because computer networks assign a numerical address (such as an IP number) to identify a device, a hostname is created for the numerical address: so as to make the hostname more memorable.
So to conclude, the host file is a plain text file which is used by hosts that connect to the Internet to locate the address of networks connected to the Internet. The host file plays an important role in translating hostnames into IP addresses, and to resolve naming errors. The host file is susceptible to security issues: if a computer system is accessed, the host file can be changed, and redirect the computer system to bogus websites etc.
The history of the host file dates back to the creation of wide area computer networks. ARPANET was one of the first wide area networks, and the first wide area network to use packet switching in North America. ARPANET was the first computer network to use a host file, and the host file included all the addresses of the nodes which were members of the ARPANET network. The host file was downloaded from a single location, the Network Information Center (NIC) located at SRI International (Stanford University, located in Menlo Park, California). Named HOSTS.TXT, the file was downloaded from SRI, and it was then stored on the operating system of the ARPANET node. Eventually, in 1984, the Domain Name System (DNS) would replace how hostname's were assigned.
Therefore, the architecture used by the ARPANET would eventually be used on the Internet. Computers that connect to the Internet still use a host file; different operating systems store the host file in a different directory and implement it in a different way. In October 1985, there was an attempt to create a standard for the host file: Request for Comment document 952 was titled "DOD INTERNET HOST TABLE SPECIFICATION". It included the location where nodes could download the host file from: SRI-NIC.ARPA (18.104.22.168 or 10.0.0.51 (SRI International) and the syntax by which a host file should be wrote.