"Tags" and "Contacts and Friends"

[Mongo]

Howdy, just a couple of questions:
1. "Tags" what is this? how used? why used? when used? etc.

2. "Contacts and Friends" So, I've had a few pick me to be their "friend". What am I supposed to do... send 'em flowers? Or is just some kinda popularity thing?

3. "Social Groups" can other people, not in your group, read your group's posts?

 

[Spark]

1: Tags
2: Contacts & Friends
3: Social Groups

In short, all of this is covered in the Frequently Asked Questions.