Keep in mind when modifying or adding new files on different branches, if there isn't a commit, those changes will “propagate” to other branches if you switch. When you’re working on a branch and you're not quite finished but need to move onto another branch, git stash will take all the changes in your working tree and index and “stashes.” You'll run git stash apply to “unstash” the changes. Keep in mind it is applied to the current branch.

When you perform a stash, it's a merge commit. Git keeps track of the state of the index/staging-area and the working tree. Keep in mind that the index and working tree could contain changes to the same file. So essentially there are two commits when you stash. With these two commits, Git is able to “unstash” your changes.

# This command will run "apply" and a "drop" to keep your stash list clean.
git stash pop

git init
Initialized empty Git repository in /home/john/Projects/git-stash-test/.git/
 
echo "bacon" > bacon.txt
git add bacon.txt
git commit -m "initial commit"
[master (root-commit) 6d2649a] initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 bacon.txt
 
ls
bacon.txt
 
echo "bits" >> bacon.txt
git stash
Saved working directory and index state WIP on master: 6d2649a initial commit 
HEAD is now at 6d2649a initial commit
 
git stash list
stash@{0}: WIP on master: 6d2649a initial commit
 
git show stash@{0}
commit d152562d7dcbf7ebdf806977cae8308f1e7bb663
Merge: 6d2649a d5b0ce0
Author: John Smith <john.smith@gmail.com>
Date:   Wed Jul 22 02:26:34 2015 -0700
 
   WIP on master: 6d2649a initial commit
 
diff --cc bacon.txt
index 6e953b2,63953b2..b11453f
--- a/bacon.txt
+++ b/bacon.txt
@@@ -1,1 -1,1 +1,2 @@@
  bacon
++bits
 
git log --oneline
6d2649a initial commit
 
git branch
* master
 
git fsck --lost-found
Checking object directories: 100% (256/256), done.
 
git show-ref
6d2649ab6b3f34ecf5437314f08f9b291f8fb9e3 refs/heads/master
d152562d7dcbf7ebdf806977cae8308f1e7bb663 refs/stash