Do not believe a thing because you read it in a book! Do not believe a thing because another has said it so! Find out the truth for yourself.

Bill Vaughan, according to the Internet

Don't believe all I say, RTFM.

Move your HEAD with Git

revert, checkout, reset

git revert

Cancels a commit by creating an opposite commit
Useful to "remove" commits already publicly published
Cancel live breaking merge

git revert

Format: git revert <git-object>

example: git revert 123abc

git revert

Revert merges

git revert

Special cases, merges have multiple parents
Which part of the merge to revert
In previous example: myBranch1, myBranch2 or both can be reverted
-m # option specify the parent to revert

Revert merges

git checkout

Different behaviour depending on arguments
git object and path can be provided or just git object

git checkout [gitObject] path

Change state of given file

git checkout [gitObject] path

Does NOT change HEAD
If no object given, discards any unstaged changes
If an object is given, changes the file to match the object. Stages the diff between the object and HEAD

git checkout gitObject

Change the position of HEAD and the state of the working tree
Links HEAD to targeted object
If object is NOT a branch, HEAD is detached
Used to switch between versions of the code
Does NOT change the state of branches

"cd" for git

git checkout gitObject

git reset

Different behaviour depending on arguments
git object and path can be provided or just git object

git reset [gitObject] path

Set the staged state of given file at the state of the given object
Sets the modified state of the file to keep the file unchanged
Corollary: If no object is provided (HEAD used by default), the command unstages the file

git reset gitObject

Moves HEAD to the provided object
If HEAD was on a branch, moves the branch tip as well
makes the commits from the branch's old position potentially unaccessible (unless using reflog, that is)
Useful to "get rid of" the n last commits of a branch

git reset gitObject

What about changes in between?

git reset --soft : keep difference staged
git reset --mixed : keep differences in working tree (default)
git reset --hard : discard differences

Conclusion

Know what you want to do to know what to do

Conclusion

If you want to:

Cancel a published commit

git revert gitObject

Conclusion

If you want to:

Go somewhere in your commit history
Go to a specific branch

git checkout gitObject

Conclusion

If you want to:

Create a new branch and switch on it

git branch theBranch

git checkout theBranch

git checkout -b theBranch

Conclusion

If you want to:

remove local modifications in a file
change a file to a desired point in the history

git checkout [gitObject] path

Conclusion

If you want to:

move the tip of a branch to somewhere else (eg remove the last commits or take the remote version of a branch)

git reset --hard newPosition

Conclusion

If you want to:

lose all local changes (staged or not)
cancel an ongoing merge

git reset --hard

Conclusion

If you want to:

unstage your changes

git reset