I've lost data a few times not thinking about what I was doing. :P
Alright, so I promised you a crack at an explanation. List, please feel free to chime in and correct me where I am mistaken.
All changes that you make to your repository are stored in what are called "commits"; these are, essentially diffs, and point to "parent" and "child" commits, so that you can trace a path along commits and see your code change along the way.
Now, branches and tags are actually "pointers to commits"[*] in a way. A branch or a tag in itself does not actually contain any changes or code. When you reference a branch or tag, you're actually referencing the commit that they point to.
When you create a tag, you give it a commit that it should point to (if you do not provide a commit, it actually defaults to where you are now), and then the tag never changes. This is why it is useful for "tagging"[*] releases. You put it there and it will stay there so that other people can see it. In this way, it is permanent[*] (unless you really want to delete it).
Branches, however, are different; whenever you checkout a branch, whatever you do, whether it may be resetting, creating new commits, or merges, the branch follows you around, such that it records your actions as being part of that branch. So a branch will "flow"[*] over time as you add more code. Thus, branches are semi-permanent[*], as the commits they point to change, but they stay in the "past" of the branch.
Things like HEAD and FETCH_HEAD are similar in that they too "point at commits", but unlike branches (which are semi-permanent) and tags (which are permanent), they are temporary. HEAD moves around; a _lot_. What HEAD really does is point to the commit that you are checked out on. Whenever you switch branches, move around, make commits, merges, and whatever else, HEAD keeps changing to point at the commit you are now on, whether it be a previously existing one, or a newly created one.
So, in your repository, you have branches, and tags, and a bunch of temporary pointers, such as HEAD. So does everybody else. How do you tell which is which?
Well, that is why when I told you to git reset, I told you to tell it to point to "origin/master". What that means is the pointer named "master" on the remote repository called "origin". (More on remote repositories later). If you had said "git reset --hard master" it actually would have reset you to _your_ master branch. If you were already checked out on master, you would've gone nowhere.
So, "git reset --hard branch-name" takes whatever branch you're on, and makes it point to the same place as branch-name, which exists in your local repository. As a corollary, "git reset --hard remote-name/branch-name" takes the branch you're on, and makes it point to the same place as branch-name on the _remote_ repository named remote-name. (More on remote repositories shortly).
If you were to do, however, "git reset --hard HEAD", you don't actually move to anywhere; but what it does do, because you've supplied the "--hard" option, is *discards* _all_ changes made from the commit that HEAD points at. This means that if you have made any changes, but have not committed them, they will be lost. This is useful if you made some changes which you decide you don't need, or if you were merely testing something in your code. But use it with caution, as you will _all_ changes; so if you made some changes, but decided you didn't want to keep some, commit what you want to keep first.
But what if you did a merge or already committed some changes, and want to undo that? Well, that is where the modifiers[*] come in. The one you've already been introduced to is the carat character (^). What this actually means is point to the commit _before_ the one referenced. So, saying HEAD^ means the commit before HEAD. And because merges are actually commits, you can easily undo a merge by going to the commit just before it. You can also chain carats together; HEAD^^^ means the commit *three* before where HEAD points. You can also use carats for other commit references[*], such as branches and tags. Want to go to the commit just before v1.0 (for whatever reason)? No problem. Just do a "git checkout v1.0^".
But, what if you want to go to the commit ten or twenty before v1.0? The first thing I would say is that is far too specific without actually knowing where you're going and you should reconsider your strategy. However, if that is what you wish to do, you could easily stack twenty carats together. You can see that this would be very unwieldy; that is why git provides us with another modifier: the tilde (~). You must always follow the tilde with a number. What it says is that you want to go X commits before the referenced one. So, "git checkout v1.0~17" will take you to the commit *17* before v1.0.
I said before that that you have your local repository and the remote repository. You can actually have any number of remote repositories referenced. Initially, though, the only remote repository you have is the one where you checked out from (unless you created a clean repo yourself). You can add and remove remote repositories through the "git remote" command.
You were afraid about not creating a branch and being sunk. The important thing to realise is that whatever you do in _your_ repository does not affect the _remote_ repository. The information in the remote repository will stay there until _they_ decide to change it. So the only way for you to affect the remote repository is for them to take what you did and apply it to their repository.
Secondly, even given that, your remote pointers (the origin/* pointers) cannot be changed by you. All you can do is remove them (you can add them back at any point; this is not a loss) and synchronise[*] them with the remote, if they've changed there.
This means that no matter what you did, unless you actually deleted the origin/master remote branch (which you could easily get back anyway), the "git reset --hard origin/master" command will always take you back to your happy place, or, more correctly, where the remote was pointing its master branch to when you last synchronised.
You can view all the remote branches you can reference by using the "git branch -r" command, without any arguments. You can also view all your local branches by using "git branch" (without the -r). It works similarly for "git tag".
I hope I've covered everything you wanted. If not, there are plenty of resources, and you can always ask here.
It also helps to visualise these things, and there are several tools that help you in this regard. The first is "git log --graph", which will show you lines on the side, showing you how commits are connected. Then, there are a number of external programs. I personally use gitk, but I know there are others, if it doesn't suit your tastes.
And have a read through the documentation. You probably won't understand most of it the first time, but the more you read it, the more you'll understand.
Good luck, Tim.
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