The Host is the domain the request is being sent to. This header was introduced so hosting sites could include multiple domains on a single IP.
The Origin header is the domain the request originates from.
The Host header is always included. The Origin header is included sometimes: It is always included on cross-origin requests (across all browsers), and in Chrome/Safari, it is also included on same-origin PUT/POST/DELETE requests. Same-origin GET requests do not include an Origin header.