Git: How to Undo commit *and* revert to last branch

Question

Uh oh... I mistakenly committed a pretty complex change (including subdirectory and files renames) without really knowing what I am doing (or what Git would be doing).

I now want to undo everything such that:

1. commit is completely reversed (as if it has never been done, perhaps removing it from history as well)
2. Restore current working directory (where .git is) to a certain branch (last one will do for now).

I found references to git reset --soft and git reset --hard but I have already proven to myself that I can do real damage by prematurely using a command without fully understanding it. :)

I found the git reset man page but I am still confused as to:

1. What is HEAD?
2. What is the difference between HEAD and * master?
3. In my situation (see above) do I need to use --soft, --hard or other (3 more options)?
4. Do I need to run another command (after doing git reset) to "finalize" the reversal?

UPDATE: After reading the answer below:

1. Do I understand correctly that all I need to do in my situation is issue a single command git reset --hard HEAD^?
2. How do I verify that reversal was performed correctly?

Answer 1

1. HEAD is the latest commit of the checked-out branch.
2. master is a branch (the main branch, by convention) whereas HEAD is a location in history for the checked-out branch. HEAD is relative to the branch you are on.
3. git reset --soft will leave your changes in the working tree, uncommitted for you to do whatever you like with. git reset --hard will restore the working tree to the state it was in at the commit you reset to.
4. No other command is needed.

First, to keep the commit in case you want to inspect it later, make a branch:

git checkout -b my_bad_commit

(or alternatively do git branch my_bad_commit as mentioned in larsman's comment.)

Then return to master or whatever branch you were on and reset:

git checkout branch_with_bad_commit
git reset --hard HEAD^

HEAD^ translates to "the parent of HEAD," which you can even stack for HEAD^^ = 2 commits back. For more on this topic, check the git community book chapter on undo in git