Sharepoint/Office365 groups. Security Vs Distribution List

I create a Group on Office365 Admin site. It is a Security Group (by default as there is no other option) This Group doesn't have an Email associated to it. Now I create a Distribution list(Exchange admin center) and when I refresh Office365 admin site page, this group starts showing with an Email address bound to it.

Now my question has several parts.

1) Is there a diff between Security Group created from office365 admin site and Exchange admin center?

2) Why is that Office365 doesn't ask for an email when I create a Group there? But when I create a security group on Exchange admin center, it asks me that!

3) Why is it that I am able to add a group to Security Groups on Office365 admin site but I can't do the same with Distribution Groups/Lists? Auto complete doesn't work for groups when I try to add a group as a member to Distribution List on Office365 admins site.

Much appreciated!