What are route parameters in Express.js?

Route parameters are named URL segments that capture values from the URL path, defined with a colon prefix. For example, app.get('/users/:id', (req, res) => { res.send(req.params.id); }) — a request to /users/42 sets req.params.id to "42". Multiple parameters: /users/:userId/posts/:postId. All parameter values are strings — parse them with parseInt() or validate/cast them in middleware. Route parameters differ from query strings (/users?id=42, accessed via req.query.id) — parameters are part of the route path and typically identify a specific resource, while query strings are optional filters.