Every HTTP request and respnse has headers. Headers are a way for the browser or server to send extra information!

Firefox: Accept-Encoding:gzip

This means “I understand compressed responses”

Headers have a name and a value.

Accept-Encoding is the name, gzip is the value.

Header names aren’t case sensitive:

aCcEpT-eNcOdIng: gzip is totally valid.

There are a few different kinds of headers:

• Describe the body:

Content-Type: image/png 
Content-Encoding: gzip
Content-Length: 12345
Content-Language: es-ES

• Ask for a specific kind of response

Accept: image/png
Range: bytes=l-10
Accept-Encoding: gzip
Accept-Language: es-ES

(Every Accept-header has a corresponding Content-header)

• Manage caches:

ETag: "abc123"
If-None-Match: "abc123"
Vary: Accept-Encoding
If-Modified-Since: 3 Aug 2019  13:00:00 GMT
Last-Modified: 3 Feb 2018 11:00:00 GMT
Expires: 27 Sep 2019 13:07:49 GMT 
Cache-Control: public, max-age=300

• Say where the request comes from:

User-Agent: curl
Referer: https://examplecat.com

• Cookies:

Set-Cookie: name=julia; HttpOnly (server -> client)
Cookie: name=julia               (client -> server)

and more!