The web or the network
Eg : Web servers and modules, database servers, DNS servers, ...
Eg : Firewall, anti-virus, anti-spam, certification authority, certificates management, secure remote access.
Eg : server monitoring, hacker attacks detection, creating graphics of server resources use, ...
Transfer or file sharing
Eg : FTP server (to upload your website or store files on your server), NFS (useful in a company), ...
E-mail server and access protocols
Eg : an email server (E-mails are stored on your server), SMTP (send), IMAP and POP3 protocols (access to e-mails).
Centralization of contacts on a server
Eg : Searching contacts on a LDAP server or synchronization with CardDAV.
Known Linux distributions
Download legally and for free Linux distributions used for servers or by individuals.
Eg : Various utilities for Linux.