Have you bought a new MacBook Air or Retina-Display MacBook Pro to replace your older MacBook? If so, you might have noticed that the power adapter that came with your new laptop has a thinner connector on it. The thinner connector is called a MagSafe 2 connector. Apple introduced these in 2012 as a replacement for its previous magnetic connector, MagSafe, introduced in 2006. If you’d like to continue to use your older MagSafe power adapter you’ll need to buy the $10 MagSafe to MagSafe 2 adapter from Apple.
[Update November 2016: Apple’s newest laptops, the MacBook Pro with TouchBar, introduced in Oct 2016 and the 12-inch MacBook, introduced in early 2015, both have USB-C connectors on them. Thus Apple’s newest power adapters no longer use MagSafe connectors at all. Instead, they have USB-C connectors.]