/etc holds system-wide configuration files. Every file in /etc is a plain text file or a directory of text files; no binaries live here. Editing a file in /etc changes the behaviour of the corresponding service or subsystem for all users on the machine. User-level configuration lives in dotfiles under $HOME, not in /etc.

/etc/fstab          filesystem mount table; read at boot
/etc/passwd         user account database (no passwords; those are in /etc/shadow)
/etc/shadow         hashed passwords; readable only by root
/etc/group          group database
/etc/hostname       the machine's hostname
/etc/hosts          static hostname-to-IP mappings; consulted before DNS
/etc/resolv.conf    DNS resolver configuration: nameserver addresses, search domains
/etc/os-release     distribution identity: NAME, VERSION_ID, etc.
/etc/fstab          filesystem table: what to mount, where, and with what options
/etc/ssh/           SSH daemon and client configuration
/etc/sudoers        rules for which users may run commands as root via sudo
/etc/crontab        system-wide cron jobs
/etc/apt/           APT package manager configuration and source lists (Debian/Ubuntu)
/etc/systemd/       systemd unit overrides and system configuration

Most services follow the convention of placing their configuration in /etc/<service>/ or /etc/<service>.conf. A file dropped into /etc/<service>.d/ typically overrides the main config without editing it directly, which makes package manager upgrades cleaner — the package can update the main file while the local override remains untouched.