The /etc directory contains a lot of files. Some of them are described below. For others, you should determine which program they belong to and read the manual page for that program. Many networking configuration files are in /etc as well, and are described in the Networking Administrators' Guide.
META: HOSTNAME, adjtime, disktab, gettydefs, networking (exports, host.conf, hosts, hosts.equiv, inetd.conf, named.*, networks, ntp.conf, protocols, resolv.conf, rpc, services, syslog.conf), mtools, and so forth.