The query string is the part of a URL after ? that carries data to the server as name=value pairs joined by &.

https://example.com/search?q=web+tech&page=2

Role in web communication:

• Passes parameters from client to server in HTTP GET requests (form input, search terms, filters, pagination).
• Lets the same resource return different content based on the parameters.
• The server reads it (e.g. $_GET['q'] in PHP, req.query.q in Express).
• URLs with query strings can be bookmarked and shared.

Limitation: data is visible in the URL and logs and length-limited, so never send passwords through it.