git reset does know five "modes": soft, mixed, hard, merge and keep. I will start with the first three, since these are the modes you'll usually encounter. After that you'll find a nice little a bonus, so stay tuned.

Let's assume you have a repository with a history akin to this:

7e05a95  (HEAD -> main) Update a
e62add5  Update b
ca9ae0a  Update a
9b6060d  Add c
eebe372  Add b
947586a  Add a

Where the latest commit (7e05a95) contains these changes:

diff --git a/a b/a
index b66ba06..28b68e2 100644
--- a/a
+++ b/a
@@ -1 +1 @@
-new content
+new new content

Now what would happen when you run git reset with the various different modes? Let's find out!

soft

When using git reset --soft HEAD~1 you will remove the last commit from the current branch, but the file changes will stay in your working tree. Also, the changes will stay on your index, so following with a git commit will create a commit with the exact same changes as the commit you "removed" before.

How would this look like in practice? Like this:

> git reset --soft HEAD^ # Assuming HEAD points at 7e05a95
> git status
On branch main
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
    modified:   a

As you see the changes in file a are on the index, and ready to be committed again.

mixed

This is the default mode and quite similar to soft. When "removing" a commit with git reset HEAD~1 you will still keep the changes in your working tree but not on the index; so if you want to "redo" the commit, you will have to add the changes (git add) before commiting.

In practice the result might look like this:

> git reset --mixed HEAD^ # Assuming HEAD points at 7e05a95
> git status
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
    modified:   a

no changes added to commit (use "git add" and/or "git commit -a")

The changes of file a are still there but they're not on the index.

hard

When using git reset --hard HEAD~1 you will lose all uncommited changes and all untracked files in addition to the changes introduced in the last commit. The changes won't stay in your working tree so doing a git status command will tell you that you don't have any changes in your repository.

Tread carefully with this one. If you accidentally remove uncommited changes which were never tracked by git (speak: committed or at least added to the index), you have no way of getting them back using git.

A practical example might look like this:

> git reset --hard HEAD^ # Assuming HEAD points at 7e05a95
> git status
On branch main
nothing to commit, working tree clean

As you can see, no changes remain. Assuming you also had some uncommitted changes in the file b these would be lost too!

> echo 'some uncommitted changes' > b
> git status
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
    modified:   b

no changes added to commit (use "git add" and/or "git commit -a")

> git reset --hard HEAD^ # Assuming HEAD points at 7e05a95
> git status
On branch main
nothing to commit, working tree clean

Bonus

keep

git reset --keep HEAD~1 is an interesting and useful one. It only resets the files which are different between the current HEAD and the given commit. It aborts the reset if one or more of these files has uncommited changes. It basically acts as a safer version of hard.

Let's revisit the example from before, where you had some uncommitted changes in b:

> echo 'some uncommitted changes' > b
> git status
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
    modified:   b

no changes added to commit (use "git add" and/or "git commit -a")

> git reset --keep HEAD^ # Assuming HEAD points at 7e05a95
> git status
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
    modified:   b

no changes added to commit (use "git add" and/or "git commit -a")

You removed the changes in file a but retained the uncommitted changes in file b!

So to reiterate: "hard" will remove all changes while "keep" only removes changes from the reset commit(s).

Each of these modes is explained in depths in the git reset documentation.

Note
When doing git reset to remove a commit the commit isn't really lost, there just is no reference pointing to it or any of it's children. You can still recover a commit which was "deleted" with git reset by finding it's SHA-1 key, for example with a command such as git reflog.

git revert just creates a new commit -- you can "remove" it with git reset --hard HEAD^ (be more careful with it, though!)

The safest and probably cleanest way to go is to rebase interactively.

git rebase -i HEAD^^

Or,

git rebase -i baf8d5e7da9e41fcd37d63ae9483ee0b10bfac8e^

From there you can squash commits, which puts one or more commits together into the previous commit. To completely delete a commit from the history, delete the line from the list.

You can revert a commit with git revert but its going to add more commit messages to the history, which may be undesirable. Use the -n parameter to tell Git not to commit the revert right away. You can rebase interactively and squash those on up to a previous commmit to keep things clean.

If the two commits you're working with here affect the same file(s), you may see a merge conflict.

Resetting the repository with git reset --hard should be done with care, as it cannot be undone.

Rewriting history should be done with care.

Pat Notz is correct. You can get the commit back so long as it's been within a few days. git only garbage collects after about a month or so unless you explicitly tell it to remove newer blobs.

$ git init
Initialized empty Git repository in .git/

$ echo "testing reset" > file1
$ git add file1
$ git commit -m 'added file1'
Created initial commit 1a75c1d: added file1
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 file1

$ echo "added new file" > file2
$ git add file2
$ git commit -m 'added file2'
Created commit f6e5064: added file2
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 file2

$ git reset --hard HEAD^
HEAD is now at 1a75c1d... added file1

$ cat file2
cat: file2: No such file or directory

$ git reflog
1a75c1d... HEAD@{0}: reset --hard HEAD^: updating HEAD
f6e5064... HEAD@{1}: commit: added file2

$ git reset --hard f6e5064
HEAD is now at f6e5064... added file2

$ cat file2
added new file

You can see in the example that the file2 was removed as a result of the hard reset, but was put back in place when I reset via the reflog.