Apple likes to keep Mac names simple but that’s not always helpful. For instance, if you want to add RAM to your Mac it’s not good enough to know that it’s an iMac. You’ll need to know that it’s a 27-inch iMac with Retina display from late 2014.
To find that out, choose "About This Mac" from the Apple menu. In some cases, you might even need to know the model identifier which is a numeric code that’s accessible if you click the System Report button in the About This Mac window and then click Hardware at the top left of the System Information window. It will be something like iMac15,1.