This blog post covers it nicely and the linked video is excellent: http://gamingduo.blogspot.com/2010/09/eve-tactics-fist.html

In summary, a fist fleet is armoured heavy assault cruisers (plate and resists) with an afterburner and mostly electronic warfare in the midslots. The idea is to be able to avoid most of the big guns with a small signature, large EHP, EW and a little speed/agility.