Here, "user/user1" is part of url which maps to the servlet for user action. In DD, you define something like this,
[Please dig up little about how to use the wild card '*'.]
In UserServlet, you can extract the part of request url, which gives you 'user1', and you can do what ever you want to do with that user. This idea is implemented here on JR, like 'My Posts' [See URL].