+Git tracks content not files
+----------------------------
+
+With git you have to explicitly "add" all the changed _content_ you
+want to commit together. This can be done in a few different ways:
+
+1) By using 'git add <file_spec>...'
+
+ This can be performed multiple times before a commit. Note that this
+ is not only for adding new files. Even modified files must be
+ added to the set of changes about to be committed. The "git status"
+ command gives you a summary of what is included so far for the
+ next commit. When done you should use the 'git commit' command to
+ make it real.
+
+ Note: don't forget to 'add' a file again if you modified it after the
+ first 'add' and before 'commit'. Otherwise only the previous added
+ state of that file will be committed. This is because git tracks
+ content, so what you're really 'add'ing to the commit is the *content*
+ of the file in the state it is in when you 'add' it.
+
+2) By using 'git commit -a' directly
+
+ This is a quick way to automatically 'add' the content from all files
+ that were modified since the previous commit, and perform the actual
+ commit without having to separately 'add' them beforehand. This will
+ not add content from new files i.e. files that were never added before.
+ Those files still have to be added explicitly before performing a
+ commit.
+
+But here's a twist. If you do 'git commit <file1> <file2> ...' then only
+the changes belonging to those explicitly specified files will be
+committed, entirely bypassing the current "added" changes. Those "added"
+changes will still remain available for a subsequent commit though.
+
+However, for normal usage you only have to remember 'git add' + 'git commit'
+and/or 'git commit -a'.
+