What each is for.
HTTP AND HTTPS
Web traffic. HTTPS is the encrypted form, and what everything should use.
SMTP
Sending mail between servers and from clients.
IMAP AND POP
Retrieving mail. IMAP keeps mail on the server; POP downloads it.
FTP, FTPS AND SFTP
File transfer. Plain FTP sends credentials readable; use the encrypted forms.
SSH
Encrypted command-line access to a server.
DNS
Name resolution.
DHCP
Automatic address assignment on a local network.
NTP
Time synchronisation.
Frequently overlooked, and a wrong clock breaks certificates and authentication.
WHAT TO REMEMBER
Each has conventional ports, and each has secure and insecure variants where applicable.
WHAT TO ALWAYS PREFER
The encrypted variant.
WHAT TO DISABLE
Insecure variants you do not need.
