OS X's Screen Sharing
All recent versions of OS X come with a suite of built-in screen sharing features. The easiest way to use them is through the Back to My Mac feature enabled under System Preferences > iCloud. However, Back to My Mac is a finicky system, andmore to the pointonly works with computers logged in to your iCloud account, so it's not really that helpful for troubleshooting other people's computers.
Ad hoc screen sharing is also pretty easy if both computers are on the same network. Just click on the other computer in the Finder's sidebar and click the Share Screen button that appears beneath the Finder windows toolbar; you'll also need a username and password for that machine (see Figure 3). Really, though, if you're on the same network, its probably easier to physically use the other machine, but just in case thats not possible, screen sharing is always a good fallback option.
Remotely viewing the screen of a computer that doesn't belong to you using OS X's built-in screen sharing is actually pretty complicated, however. While it's doable using techniques like port forwarding, it requires some technical know-how that is beyond the scope of this article.