1) Clans have special roleplay purposes, and have outfits I believe. Mostly just for roleplay, not necessary to the gameplay, but it's nice to have a "family" upon joining.
2) Villages are very important. There are village relations, you can fight enemies, war, different geographic locations, different skills to obtain. Leaf and Sand appear to be the most full atm.
3) I'm not sure, you may find it best to look around. I know if you go to a scroll scribe you can look at all the jutsu.
4) Summons are earned through special village missions I think. It's not a mastery.
5) Dunno, but a lot of people from around the world play!