Originally, "Feed the Beast" was a challenge map created by well-known Minecrafters (Slowpoke et al, I believe) incorporating many well-known mods, such as IC2 (and its addons CompactSolars and AdvancedMachines), RedPower, WR-CBE, the Portal Gun Mod, BuildCraft, Equivalent Exchange 2 and Forestry. The concept was that the player entered a pyramid made of Basalt (a dark material which RedPower uses to generate volcanoes) and was required to create various highly advanced machines and dropping them into a Transposer to pick them up (thereby "feeding" the "beast") at which point they were sent through a series of complex logic gates and machines to determine if A ) the item was a valid item completing an objective specified by the "beast" and B ) which objective it completed.
Dependant on the item that the "beast" had been "fed", the player would then receive some sort of item(s) to assist the player with further progression, with the end goal being the ability to create the Omega Bee, a rare and valuable bee species available only in an FTB-specific mode of Forestry that utilized the high-tier Dark Matter and Red Matter tech tiers added by Equivalent Exchange 2 (since rewritten to Equivalent Exchange 3, where these items do not yet exist and the mod is more balanced) to cause mutations in a bee's genome that would allow for easy diamond production.
Intended as a time-sink and a challenge, the map then branched out into "Insanity" mode, a much more difficult mode, as well as a mod pack, which is what Feed the Beast is more often known as today. See below.