Epitope tags - What is T7 tag? (Apr/18/2006 )

Hi,
new to this field...................
Can anyone explain me about the different kinds of epitope tags used?
What r "T7 tags"??????????
How r T7 antibodies made???

relevant informations at abcam here : http://www.abcam.com/index.html?c=596


T7 epitope tag yeast vector
Vectors for expressing T7 epitope- and His6 affinity-tagged fusion proteins in S. cerevisiae.
pubmed T7 epitope citations