What is a gateway, which gateways matter for cyber security architecture, and which typical ones exist?

A gateway is a handover point or passage that connects two systems. For security architecture, the ones that matter carry an extra control function: email gateways, web proxies, data gateways and API gateways (each with content control), reverse proxies and application gateways.

* Every one of these is a door in the perimeter that also inspects what passes. *

• Email gateway with content control: filters mail in and out.
• Web proxy with content control: filters web access from inside.
• Data gateway with content control: business data exchange with partners.
• API gateway with content control: publishes software interfaces to the outside.
• Reverse proxy: sits in front of servers and accepts requests on their behalf.
• Application gateway: a loosely used name, often a multi-function gateway.

A web application firewall belongs to the same family but is treated together with DMZ designs. The common thread: each one is a door in the perimeter that also inspects what passes.

Go deeper:

• Application-level gateway — Wikipedia — the narrower, protocol-specific meaning of the term.
• Proxy server — Wikipedia — forward and reverse proxies, the gateways that inspect what crosses.