Git Reference

Git community book online: http://book.git-scm.com/index.html
creating:init, branch
adding and removing files::add, rm
seeing activity:log, status
basic repository operations are: push, pull, commit, checkout, clone, fetch, merge
undoing changes: reset, checkout, revert
see files in the repo:ls-files
finding stuff:grep
labeling:tag

Some git cheat sheets:
http://help.github.com/git-cheat-sheets/
http://cheat.errtheblog.com/s/git

How do I see the differences between file versions?
git diff <commit hash> <filename>

How do I go back a to version x?
Then to revert a specific file to that commit use the reset command:
git reset <commit hash> <filename>

You may need to use the --hard option if you have local modifications.
A good workflow for managaging waypoints is to use tags to cleanly mark points in your timeline. I can't quite understand your last sentence but what you may want is diverge a branch from a previous point in time. To do this, use the handy checkout command:
git checkout <commit hash>
git checkout -b <new branch name>

You can then rebase that against your mainline when you are ready to merge those changes:
git checkout <my branch>
git rebase master
git checkout master
git merge <my branch>

How do I find/look at a files history (log)?
git log <filename>
http://book.git-scm.com/3_reviewing_history_-_git_log.html

Using --stat with log will show what files changed and by how much.
git log --stat

There is also a --pretty option that provides several nicer ways of presenting the results
git log --pretty=oneline
git log --pretty=short
git log --pretty=format:'%h was %an, %ar, message: %s'

You can also use 'medium', 'full', 'fuller', 'email' or 'raw'. If those formats aren't exactly what you need, you can also create your own format with the '--pretty=format' option (see the git log docs for all the formatting options).

How do I roll back/throw away current changes?
Use checkout if you haven’t committed yet.
$ git checkout -- hello.rb
$ git checkout HEAD hello.rb
http://book.git-scm.com/4_undoing_in_git_-_reset,_checkout_and_revert.html

Use revert to fix committed mistakes.

You have to be careful when you say "rollback". If you used to have one version of a file in commit $A, and then later made two changes in two separate commits $B and $C (so what you are seeing is the third iteration of the file), and if you say "I want to roll back to the first one", do you really mean it?
If you want to get rid of the changes both the second and the third iteration, it is very simple:
$ git checkout $A file

and then you commit the result. The command asks "I want to check out the file from the state recorded by the commit $A".
On the other hand, what you meant is to get rid of the change the second iteration (i.e. commit $B) brought in, while keeping what commit $C did to the file, you would want to revert $B
$ git revert $B

Note that whoever created commit $B may not have been very disciplined and may have committed totally unrelated change in the same commit, and this revert may touch files other than file you see offending changes, so you may want to check the result carefully after doing so.

Disclaimer: I created the layout of this document, along with the questions I wanted answered for myself.  The answers are gleaned and lightly edited from results I found during the research process.  Quite a few answers were found on stackoverflow.com