See the changes

Make some changes to the config.pl file and stage it.

$ notepad config.pl
$ git status

# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#       modified:   README.txt
#       modified:   config.pl
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       setup.pl
no changes added to commit (use "git add" and/or "git commit -a")

$ git add config.pl
$ git status

# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       modified:   config.pl
#
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#       modified:   README.txt
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       setup.pl

So what was changed?

$ git diff

diff --git a/README.txt b/README.txt
index 62567d0..fb89137 100644
--- a/README.txt
+++ b/README.txt
@@ -1,2 +1,4 @@
 Hello Git
 Second line
+Third line

Only the changes to the not staged files are shown

$ git diff --cached   ( or --staged)

diff --git a/config.pl b/config.pl

index f9d55cd..e2b7f47 100644
--- a/config.pl
+++ b/config.pl
@@ -1 +1,2 @@
 # this is the config.pl file
+# second line

Only the changed to the staged files are shown

$ git diff HEAD

diff --git a/README.txt b/README.txt
index 62567d0..fb89137 100644
--- a/README.txt
+++ b/README.txt
@@ -1,2 +1,4 @@
 Hello Git
 Second line
+Third line
+
diff --git a/config.pl b/config.pl
index f9d55cd..e2b7f47 100644
--- a/config.pl
+++ b/config.pl
@@ -1 +1,2 @@
 # this is the config.pl file
+# second line

changes between working copy and HEAD