If you share a Mac with family or roommates, you're going to want to set up multiple macOS user accounts. Each account has its own documents, browser history, and saved passwords.
Setting up multiple accounts is a good idea from a security perspective even when sharing your computer with people you trust. Whenever the Mac boots, or wakes up from sleep, or returns from being locked, people can decide which account to open. Thought it's best to lock your Mac whenever you step away from your computer, sometimes that doesn't happen. So, how are people supposed to find their own account if another account is still signed in? The answer is to enable the Fast User Switching menu bar icon.
The Fast Switching icon is one of the built in macOS menu bar icons, and you should enable it on any computer with multiple users. By default this menu bar item shows the current user's full name near the top-right corner. It makes switching accounts as easy as clicking the account to which you want to switch, and then entering the password.
If you don't see anything the icon, it's simple enough to enable it. Head to System Preferences, and then open the "Users & Groups" panel.
In the "Users & Groups" panel, click the "Login Options" option. Settings here will be locked (grayed out), so click the lock icon at bottom-left and enter your password. Then turn on the "Show fast user switching menu as" option.
By default, your menu bar shows the full name of the current user, but you can use the dropdown on that option to show just the icon instead and save a little space on your menu bar.
Now anyone who sits down at the Mac can quickly access their profile. And it gets even better if you have a Mac with Touch ID: you can switch users instantly with your fingerprints. Seriously, here's how that looks:
Currently, Touch ID is only offered for laptops, which probably aren't likely to be shared by multiple people. Hopefully this feature comes to the iMac and Mac Mini later, however, because it really makes switching users seamless. Until then make sure the icon is enabled.