From the UNIX man page:
The hosts file is a local database that associates the names of hosts with their Internet Protocol (IP) addresses. The hosts file can be used in conjunction with, or instead of, other hosts databases, including the Domain Name System (DNS), the NIS hosts map and the NIS+ hosts table.
The hosts file has one entry for each IP address of each host. If a host has more than one IP address, it will have one entry for each, on consecutive lines. The format of each line is:
IP-address official-host-name nicknames...
Items are separated by any number of SPACE and/or TAB characters. The first item on a line is the host's IP address. The second entry is the host's official name. Subsequent entries on the same line are alternative names for the same machine, or "nicknames." Nicknames are optional.