FTB is stupid good. There is a metrick Fucktonne of stuffs to do in that. But instead of me trying to poorly explain it with words, allow me to link a vidya -
Well, there is no good video that just explains what exactly it is, but the wikia I linked to has a bunch of good information on the rundown for what it is overall.
Quick quote taken from the front page -
"Feed the Beast is a modpack for Minecraft. It adds a huge number of items, recipes and world-generation to the game, and is used in the YouTube series "Feed the Beast." The modpack centers on several famous mods such as IndustrialCraft and BuildCraft but the full list is extensive.
All of the mods in the pack have been approved by the modders to be included. The FTB launcher was first released on Saturday, Nov. 10, 2012, with a live-stream broadcast on the Feed The Beast website.
The launcher currently features eight different modpacks for you to enjoy, including a Beta version of the Feed the Beast modpack. The official modpack release is TBD and will most likely launch once Equivalent Exchange 3 is fully updated and stable."