Clicking a link in the left frame - the tree - is completely a client-side event. With the target set to the right frame and a link like /somepage.aspx?id=someid, it will cause the browser to request /somepage.aspx from the server, passing the id=someid as a query string. In that page's code - perhaps in the Page_Load handler - you would do something like this:
private void Page_Load(object sender, EventArgs e)
{
// No error-checking below - you should add some.
int id = Int32.Parse(Request.Params["id"]);
// Use this ID to select something from you database or a file or something
// and output to the page or bind them to controls.
}
This is very standard CGI-like handling where you request a page with a query string or post form data (corresponds to GET and POST form actions respectively) and that page uses the query string or form parameters to get information and display it.
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