The JSTL <c:url> tag is used to generate a URL based on a URL pattern and a set of query parameters. This tag is useful for creating links that include query parameters, and for ensuring that URLs are correctly encoded and formatted.

The basic syntax for the <c:url> tag is as follows:

<c:url value="urlPattern" var="variableName" />

The value attribute is used to specify the URL pattern, which can include placeholders for query parameters. The placeholders are identified by the prefix ?, and the actual query parameter names are specified using the param attribute of the <c:url> tag. The var attribute is used to specify the name of the variable to which the generated URL will be assigned.

Here's an example that shows how to use the <c:url> tag to generate a URL with query parameters:

<c:url value="/myPage.jsp">
  <c:param name="param1" value="value1" />
  <c:param name="param2" value="value2" />
</c:url>

In this example, the URL pattern is "/myPage.jsp", and the query parameters are specified using the <c:param> tags. The generated URL will include the query parameters "param1=value1" and "param2=value2".

Note that the <c:url> tag can also be used to generate URLs that include session ID information, which is necessary for maintaining sessions in some web application servers. To include the session ID in the generated URL, set the includeContext attribute of the <c:url> tag to "true", like this:

<c:url value="/myPage.jsp" includeContext="true" />

The <c:url> tag is a powerful tool for generating URLs in JSP pages, and can help ensure that URLs are correctly encoded and formatted. However, it should be used carefully to avoid generating overly complex or lengthy URLs.