Difference between __gshared and shared.

[wobbles via Digitalmars-d]

After reading the recent "Lessons Learned" article [1], and 
reading a few comments on the thread, there was a mention of 
using __gshared over shared.

What exactly is the difference here?
Are they 2 keywords to do the same thing, or are there specific 
use cases to both?
Is there plans to 'converge' them at some point?
[1] 
https://www.reddit.com/r/programming/comments/3cg1r0/lessons_learned_writing_a_filesystem_in_d/