What's the difference between “local” and “localhost” connection types in pg_hba.conf?

Question

This is the relevant part of my pg_hba.conf:

# TYPE  DATABASE        USER            ADDRESS                 METHOD

# \"local\" is for Unix dom         


        

Answer 1

local is a connection type used in pg_hba.conf, while localhost is the network address for local loopback and translates to the IPv4 address 127.0.0.1, or IPv6 ::1.
I quote the manual about pg_hba.conf:

local

This record matches connection attempts using Unix-domain sockets. Without a record of this type, Unix-domain socket connections are disallowed.

host

This record matches connection attempts made using TCP/IP. host records match either SSL or non-SSL connection attempts.

Note: Remote TCP/IP connections will not be possible unless the server is started with an appropriate value for the listen_addresses configuration parameter, since the default behavior is to listen for TCP/IP connections only on the local loopback address localhost.

For the GUC* listen_addresses in postgresql.conf, localhost also serves as setting:
_{* GUC: Grand Unified Configuration}

The default value is localhost, which allows only local TCP/IP "loopback" connections to be made.

Bold emphasis mine.