What's an OS?
OS is an abbreviation for the word Operating System. An Operating System is the software that tells a computer how to operate. It controls hardware, executes programs, manages tasks and resources, and provides the user with an interface to the computer.
What's the version of my OS?
• Windows 10 or Windows Server 2016 - Go to Start, enter About your PC, and then select About your PC. Look under PC for Edition to find out your version and edition of Windows.
• Windows 8.1 or Windows Server 2012 R2 - Swipe in from the right edge of the screen, tap Settings, and then tap Change PC settings. (If you're using a mouse, point to the lower-right corner of the screen, move the mouse pointer up, click Settings, and then click Change PC settings.). Next, Tap or click PC and devices, and then tap or click PC info. Look under Windows for the version and edition.
• Windows 8 or Windows Server 2012 - On the Start screen, type Computer, press and hold or right-click Computer, and then tap or click Properties. Then look under Windows edition for the version.
• Windows 7 and Windows Server 2008 R2 - Click Start or the Windows logo right click Computer then click Properties. Look in System.
• OS X (Mac OS X) - Click the Apple menu at the top of your screen then click About this Mac. To get more detailed information after that click More Info -> Software. Note: Apple has changed the name of it's OS to OS X from Mac OS X.
• iOS (iPhoneOS) - On your iPhone touch Settings -> General -> About. Look under version. Note: Apple has changed the name of it's OS to iOS from iPhoneOS.
Go to the Home Screen (push the home button or just keep pressing the left facing triangle (back button)). Then push the Applications button (looks like little squares or like 3 colons :::). Find and touch the Settings icon. Scroll to the bottom of the list. then touch About Phone. Look for lines that say Firmware version or Android version.
Do I have a 32-bit or 64-bit OS?
Follow the instructions above in the What's the version of my OS? section to find out if your OS is 32-bit or 64-bit. The page with the version info usually has something that says if it is 32-bit or 64-bit. Only Windows XP and later could possibly be 64-bit. If you don't see something that says "64-bit" somewhere on the version page, it is likely you have a 32-bit version of Windows. Also, more info here or here.
Mac OS X - Click About this Mac from the Apple menu at the top of your screen. Then click More Info -> Software.
Mac OS X 10.5 (or greater) can run 64-bit applications. Any processor that says: Intel Core 2 Duo, Intel Xeon, or PowerPC G5 are 64-bit processors. All currently shipping Macs ship with OS X and a 64-bit processor. 10.6 still boots a 32-bit kernel by default. It's possible to boot at 64-bit kernel. Read all about it here.