Tekkit is not a mod, but actually a bunch of different mods compiled into a modpack. It has IC2, RP2, Buildcraft, and NEI just to name a few. You can find the official site here. Along with various other modpacks.
Tekkit is pretty much an industrialized Minecraft
It can make your mind think abit considering the amount of machines, cables, pipes, and such that there are
you learn to hook them up to work together, some machines need power to run greater than redstone can use (redstone is much better in tekkit)
tons of materials, tons of new items and ores, even uranium to make a nuke (not kidding) + nuclear reactors for power and solar power
it gets to be easy after you have a few of the high level items, if you play your cards right, you will never need to mine or woodcut again pretty much, but it is your choice of action to take for efficientcy, confusing... so confusing in the beginning
So pretty much once you get some supplies, you can make item (sort of) generator, quarries, rings/ammies that give bonus's like flight or no damage from lava, teleporters and there are much more, but thats what I feel liek explaining
FTB started as a challenge map using almost the exact same mod list as Technic, which had been around longer and originally started as a private mod pack. Tekkit was the multiplayer version but due to multi and single player being merged in I think the 1.3 update they no longer update Technic. So no Tekkit is not like FTB, FTB is like Tekkit.