How is a URL structured and what components does it have?

A URL is a layered address: which protocol to speak, which host to reach, optionally which port, and which path and file to fetch on that host.

URL (Uniform Resource Locator) is the full address that pinpoints one resource on the web. Take this example:

http://www.greenpeace.org/aboutUs/overview.html
\__/   \_______________/ \__________________/
protocol   domain / host      path + filename

Its formal syntax, with the optional part marked:

<protocol>://<domain-name>:<port>/<path>/<filename>
                          \__ optional __/

Why the port is usually invisible: each protocol has a default port — 80 for HTTP, 443 for HTTPS — so the browser fills it in automatically and you only write it (e.g. :8080) when the server listens somewhere non-standard.

Go deeper:

• What is a URL? (MDN) — walks the same anatomy and adds the query string and fragment parts.
• URL (Wikipedia) — the formal scheme://authority/path?query#fragment syntax with a diagram.