Request headers take the form of key-value pairs, and represent specific aspects of the request. For example:

Accept-Encoding: gzip

Indicates that the browser knows how to accepted content compressed in the Gzip format.

Note that request headers are a subset of message headers that apply specifically to requests. There are also message headers that apply only to HTTP responses, and some that apply to both.

As HTTP is intended as an extensible protocol, there are a lot of potential headers. IANA maintains the official list of message headers as well as a list of proposed message headers. You can also find a categorized list in the MDN Documentation

While there are many possible request headers, some of the more commonly used are:

Accept Specifies the types a server can send back, its value is a MIME type.

Accept-Charset Specifies the character set a browser understands.

Accept-Encoding Informs the server about encoding algorithms the client can process (most typically compression types)

Accept-Language Hints to the server what language content should be sent in.

Authorization Supplies credentials to authenticate the user to the server. Will be covered in the authentication chapter.

Content-Length The length of the request body sent, in octets

Content-Type The MIME type of the request body

Content-Encoding The encoding method of the request body

Cookie Sends a site cookie - see the section on cookies later

User-Agent A string identifying the agent making the request (typically a browser name and version)