I just fetched from origin and git status reports:

Your branch is behind origin/ by 13 commits, and can be fast-forwarded

Strictly speaking, what does this exactly mean? Does it mean:

1. that HEAD is behind the corresponding remote-tracking-branch (origin/<branch_name>).

2. or that <branch_name> is behind the corresponding remote-tracking-branch (origin/<branch_name>)?

What if I am on a detached HEAD? (i.e. if my HEAD and <branch_name> don't point to the same commit).

Is there a way to visualize these pointers (HEAD, <branch_name> and origin/<branch_name> on a graph? I have tried with git log --graph, but I think I only see the different commits and they merge overtime.

Let's look at an example. This repository has a remote called origin with a branch called master. I also have a branch called master which I currently have checked out.

% git log --decorate --graph --pretty=oneline --abbrev-commit --all
* 072a57a (origin/master, origin/HEAD) C
* 87011c4 B
* d3c4a48 (HEAD, master) A

When I run git status it tells me that my branch master is behind origin/master by two commits.

% git status
# On branch master
# Your branch is behind 'origin/master' by 2 commits, and can be fast-forwarded.
#
nothing to commit (working directory clean)