What is ?N=D after a domain name in a search engine listing

?N=D as part of the URL indicated that values are being passed to the
web server, for example the URL to this question

http://answers.google.com/answers/threadview?id=318889#a

indicates that after the URL 
http://answers.google.com/answers/threadview

the parameter id is being passed with the value 318889

Because many web servers generate pages dynamically, even though the
page itself is of fairly static content, many search engines index
pages with parameters in the URL.

"If you decide to use dynamic pages (i.e., the URL contains a '?'
character), be aware that not every search engine spider crawls
dynamic pages as well as static pages. It helps to keep the parameters
short and the number of them small."
://www.google.com/webmasters/guidelines.html

Owain

To complete the previous comment, the N=D parameter/value are
typically used on Apache Web Server directories/files listings to
request a specific sort method.

In your case, N stands for Name and D for Descending.

N=A would do an Ascending sort by Name. Other parameters from the
Apache directory listing are:

M for Last Modified
S for Size
D for Description

Try browing this directory listing and change the sort method on my
webserver by clicking the columns headers:
http://www.tootella.org/prelude/rpms/