Questions based on tenants in SP 2013 Office 365

I have s set of question based on tenants answer of which is not correctly known till now. Please reply inline.

What is the proper meaning of tenant in SP 2013 Office 365?
Do we need a tenant to implement
    SharePoint hosted app?
    Auto hosted app?
    Provider hosted app?
Do we need tenant in development environment?
Does MS create tenant for you in office 365? I guess yes.
At what scope MS create client? 

Please explain inline and in details.

Thanking you.