Explainer1 min readUpdated June 29, 2026

What is an HTTP header?

Short answer

An HTTP header is a name/value line that carries metadata alongside a request or response, such as the content type, caching rules, or authentication. Headers describe the message; the body carries the actual data.

Metadata for the message

Every HTTP request and response can include headers: simple Name: value lines sent before the body. They tell each side how to interpret the message, who is making it, and how it may be cached or secured.

Content-Type: application/json
Cache-Control: max-age=3600
Content-Length: 348

Some response headers

Request vs response headers

Request: Authorization (credentials), Accept (formats wanted), User-Agent (the client)
Response: Content-Type (the format returned), Set-Cookie, Cache-Control
Either: Content-Length, custom X- headers

Header names are case-insensitiveContent-Type and content-type are the same header. Values, however, are usually case-sensitive.

Try it: HTTP Header InspectorInspect the headers a URL returns, locally in your browser.Open tool

Some headers are specifically about safety. See What are HTTP security headers?.

References

Questions

What is the difference between a header and the body?

Headers carry metadata about the message, like its type and length. The body carries the actual content, such as the HTML page or JSON data.

What are X- headers?

Historically, names starting with X- marked non-standard or custom headers. The convention is discouraged now, but many such headers, like X-Frame-Options, remain in wide use.

Does this send my data anywhere?

No. Utilumo's developer tools parse and transform input inside the browser tab. Nothing is uploaded, stored, or logged.