Full documentation is here: https://www.kernel.org/pub/software/scm/git/docs/gitrevisions.html
Suppose you have a repo that looks like this:
base - A - B - C - D (master)
\
\- X - Y - Z (myBranch)
Verify the repo status:
> git checkout master
Already on 'master'
> git status ; git log --oneline
On branch master
nothing to commit, working directory clean
d9addce D
110a9ab C
5f3f8db B
0f26e69 A
e764ffa base
and for myBranch:
> git checkout myBranch
> git status ; git log --oneline
On branch myBranch
nothing to commit, working directory clean
3bc0d40 Z
917ac8d Y
3e65f72 X
5f3f8db B
0f26e69 A
e764ffa base
Suppose you are on myBranch, and you want to see only changes SINCE branching from master. Use the two-dot version:
> git log --oneline master..myBranch
3bc0d40 Z
917ac8d Y
3e65f72 X
The three-dot version gives all changes from the tip of master to the tip of myBranch. However, note that the common commit B is not included:
> git log --oneline master...myBranch
d9addce D
110a9ab C
3bc0d40 Z
917ac8d Y
3e65f72 X
PLEASE NOTE: git log and git diff BEHAVE DIFFERENTLY! The behavior is not exactly opposite, but almost:
> git diff master..myBranch
diff --git a/rev.txt b/rev.txt
index 1784810..e900b1c 100644
--- a/rev.txt
+++ b/rev.txt
@@ -1 +1 @@
-D
+Z
> git diff master...myBranch
diff --git a/rev.txt b/rev.txt
index 223b783..e900b1c 100644
--- a/rev.txt
+++ b/rev.txt
@@ -1 +1 @@
-B
+Z
So, the two-dot version shows the diff from tip of master (i.e. D) to tip of myBranch (Z). The three-dot version shows the difference from the base of myBranch (i.e. B) to the tip of myBranch (Z).