Welcome to today's Express.js lesson about Route and Query Parameters. Both play pivotal roles in handling HTTP requests in web applications. Imagine sending a letter; the street address (Route Parameter) is essential, while "Attn: Tim" (Query Parameter) is optional. Now, let's dive into these exciting terms.

In web requests, route parameters come directly in the URL, serving as "placeholders" for actual values — they guide the requests, much like a GPS guiding a vehicle. On the flip side, query parameters send extra, optional information. They come after a ? in a key=value format.

'http://website.com/users/123/photos?year=2021'

In the above URL, 123 and photos denote route parameters, while year=2021 implies a query parameter.

In Express.js, we create Route Parameters in the path of the route using a colon :. Use req.params to fetch them in the route handler function:

const express = require('express');
const app = express();

// Using 'userid' as the route parameter
app.get('/users/:userid', function(req, res) {
  // Fetch the 'userid' parameter from req.params object
  // and send it in the response
  res.send("The user id is: " + req.params.userid);
});

In this example, /users/123 would output The user id is: 123.

Query parameters are added after a ?. To access them, use req.query.

const express = require('express');
const app = express();

// Respond with the query parameter 'q'
app.get('/search', function(req, res) {
  // Fetch the 'q' parameter from req.query object
  // and send it in the response
  res.send("Search results for: " + req.query.q);
});

A request to /search?q=coding will give Search results for: coding.

Route parameters are the must-have data; query parameters are the good-to-have bits. Route parameters are excellent for identifying data like IDs, and query parameters work well for optional user queries or preferences.