Query String
A query string is the part of a URL after the question mark, used to pass extra information to the page.
In plain English
In a shop address with a question mark followed by colour and size values, everything after the mark is the query string. It is a list of name and value pairs joined together.
The page reads those values and changes what it shows, filtering products, remembering a campaign source, jumping to page three of the results.
Why it matters
Query strings are everywhere once you notice them: search results, product filters, pagination, campaign tracking, payment returns.
They also cause real problems, duplicate content when the same page exists at five addresses, and cache misses that slow a site down. Knowing what they are makes both diagnosable.
How it works
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A question mark starts itEverything before it is the path.
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Pairs are joined togetherEach is a key and a value, in any order.
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The page reads themAnd responds accordingly.
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Special characters get encodedA space becomes a percent code or a plus sign.
Nothing private goes in an address
Query strings are stored in many places along the way. Secrets and personal data belong in a request body instead.