I don’t necessarily recommend that everyone start doing this, as I’m a big fan of having separate passwords for everything (and using 1Password to manage them). That said, if you’re helping someone who’s less tech-savvy (or if you have difficulty tracking your own passwords), this could be a way to simplify things.
To get started, first open System Preferences from under the Apple Menu in the upper-left corner of your screen.
When that launches, choose the “Users & Groups” pane from the list (its icon looks like a pair of silhouettes).
Click the lock in the lower-left corner and type in your administrator password to unlock the pane.
Next, choose the “Change Password” button near the top of the window, and after you do, you’ll see the options that Yosemite has to offer!
As you can see, you could just change your login password here, but if you select “Use iCloud Password,” your Mac will walk you through the steps for doing just that. Next will be a screen asking you to enter your old password (which is the one you currently use for logging in to your Mac) and your iCloud password.
After typing that info in, click “Use iCloud Password,” and you’re done. You can tell this is turned on—your iCloud account’s email address will appear under your username within that pane.
Trust me, it’s there.
From that point on, your iCloud password and your Mac’s user account password will be the same, so you’ll use that info both for logging in at iCloud.com and for installing applications on your computer (or whatever).
Finally, if you’d like to undo this at any point, it’s just as simple. Revisit System Preferences> Users & Groups, unlock the pane as before, click the “Change Password” button, and you’ll be given the option to revert to using a separate password again.
To me, this is very cool but a bit hard to get used to. In the testing for this article, I turned the feature on and changed my iCloud password, and voilà—my Mac’s password immediately changed, too, which is nifty. But strange. But nifty. I’m actually not sure which of those opinions I hold more strongly, to be honest.