hi
IFS - Inter Frame Space.
It is used to coordinate access to transmission medium. Vrying interframe spacings create different priority levels for diff types of traffic.
You can refer the detailed description about IFS in the book
"802.11 Wireless Networks: The Definitive Guide" By Matthew Gast. ( download the book from free ebook club - www.gigapedia.org)
and can also refer "A Technical tutorial on IEEE 802.11 standard". (get it from google search by using keyword as IFS in 802.11)