An act= in JFB stands for 'action'. Generally these links are associated with features that tell your board what page to call up. For example, act=Arcade brings up the arcade. act=Downloads shows the downloads.

Help = /index.php?act=Help
Search = /index.php?act=Search&f=
Members = /index.php?act=Members
Calendar = /index.php?act=calendar
Store = /index.php?act=store
Downloads = /index.php?act=Downloads
Affiliates = /index.php?act=daffiliates
Shoutbox = /index.php?&act=Shoutbox
Army = /index.php?act=army
Lottery = /index.php?act=lottery
Bookie = /index.php?act=Bookie
Usergroups = /index.php?act=usergroups
Reputation = /index.php?act=rep&CODE=totals
Java IRC = /index.php?act=javairc
Arcade = /index.php?act=Arcade
Rules = /index.php?act=boardrules
Index = /index.php?act=idx