Apple currently produces two operating systems, each of which receives separate updates.  To see which version you are currently using, and to check if updates to this operating system are available, follow the instructions below:

Mac OS X:

OS X is the operating system used on Apple laptops (such as MacBooks) and desktops (such as the iMac).  To check the version your device is using, click the "apple" icon at the top left of the screen, then click "About this Mac".  To check to see if there is an update available for your computer, simply click "Software Update" in this window.  

iOS:

iOS is the operating system used by iPhones and iPads.  To check the version your device is using, tap the "Settings" app on your device, then select "General", then "About".  To check if an update is available for your device, press the "back" button to return to the "General" settings page, then tab "Software Update".