how to put two controllers/methods on one url

I'm a basecamp user. At basecamp URl such as
http://myproject.projectpath.com/projects/488470/milestones/new, both
projects and milestones are controllers (are they?), 488470 is project
id and new is method of milestones. Usually, you get
http://hostname/controller/method/otherinfo, but how they put their url
together and let rails now how to interprete the URL? via route? Can
any body gives some details?

Thanks a lot!