What are these pins for on the Arduino?

Question

Arduino

Good evening.
Working with my Arduino Uno, I found that some contacts were completely unfamiliar to me, and after that I discovered a lot of interesting things:

1 - On the back there are SDA and SCL inscriptions, i.e. are these the same 4.5 analog pins? or other?
2 - ICSP for the USB interface, as I understand it, there are 4 more holes under it, what are they for?
3 - RE ET EN, Google did not give anything, there are 2 bare connected contacts, what are they for?
4 - What is it for?
5 - Also 2 bare connected contacts, what are they for?
I would like to know what and with what these contacts and connectors eat? =)