To get the entire path use the function Composite.Web.Server.ServerVariable( Variable name = "URL" )
To get a value from the "path info" use the function Composite.Web.Request.PathInfo
To get a value from the query string use the function Composite.Web.Request.QueryString
Components in a URL are like this
http://server/PATH_TO_PAGE/PATH_INFO?QUERY
Typically page requests are just http://server/PATH_TO_PAGE - PATH_INFO and QUERY only gets into the mix if you actively print out page URL's where you add this.
Example of a PATH_INFO string could be: "/hello/world"
Example of QUERY could be "?id=39".