The
Quick Start page gives an overview what's possible. For logging in, check out the section "Working With Forms" (assuming that's how the login works). If the login sets a cookie, jWebUnit will remember that, and use it from then on.
If you need the complete page source (as opposed to specific elements), you can get that using the getPageSource method in the class that extends WebTestCase. You can also use saveAs to save the last accessed page to a file. The getElementTextByXPath and getElementAttributByXPath methods help getting at particular parts of the page.
And, yes, the library is meant for testing web apps, but it's superb for automating any kind of access to a web app by any Java client.