1) It is required to play Minecraft. All the information you type in redirects you to minecraft.net and then it downloads the mods and installs them. You can check the code because it is opensource.
2) Different modpacks for different gameplays. Technic is for SSP, Tekkit has Technic SMP mods only, Yogbox is a fun RPG like modpack, Voxelbox is for the creative ones, Hack'n'Slash is a full dungeon RPG modpack and then you have Vanilla.
3) No idea.