what's differential antenna?

I am a newer in the field of antenna design. who can tell me what's a differential antenna? and what's a single ended antenna?
in which book i can find the priciple of these kind of antenna?
thanks1

Take a look at the pictures below ..
For more details try:
"Single-ended or differential?
That is the question"
http://www.edn.com/contents/images/272745.pdf

Regards,
IanP