Fleet carriers have a larger number of fighters, and enough armament and support equipment to service and keep those fighters flying for an extended period compared to other ships, from the way the books described them; the Tiger's Claw was a Strike carrier, as was the Eagle's Talon and the Kipling, and they were considered fleet carriers, IIRC. Most all of the fleet carriers we've seen (save the older TCS Victory) had a large number of fighters - 104, then 120, and then 96, plus fairly heavy armament for a craft of her type.. or at least I don't recall light carriers having the same amount of guns or torps.
As LOAF indicated, Fleet Carriers are the center of task forces or carrier groups - so that's a role more than anything. However, at least up until WC3, these carriers were also fairly formidable craft in their own right, able to support a large number of strike craft. Maybe the terms 'heavy carrier' and 'supercarrier' are probably more applicable to the type of fleet carrier described earlier on in the thread.