When a customer buys a new Mac, there’s often a question of what to do about the old data. All that old data is all that stuff that made your old machine yours: it’s the settings, the pictures, saved web page bookmarks, documents, spreadsheets…all that stuff.
Frequently a Mac-to-Mac data transfer can be done without any special equipment or advanced knowledge. The easiest way is to use Apple’s Migration Assistant which is a program built into OS X, and is on every new Mac. When setting up a new Mac (or any Mac that’s been reset to factory settings, generally from the disk being wiped and the OS being reinstalled) it’ll prompt you to make a decision.
The top option is to transfer data from a start up disk or Time Machine backup. If you’re already doing a Time Machine backup to an external HDD this is the best option, just make sure your backup is completely up to date. If it’s behind, any changes you’ve made won’t show up on your new machine when the transfer completes. Once you’ve identified the drive you want the data to come from it goes through and calculates the sizes of everything on that older drive. You’ve got a little control of what comes over, like whether or not you want the entire Applications folder, but nothing more specific than that. It’ll also tell you how much available space will be left over, or if there’s more data on the source drive than the destination.
I recommend using a Time Machine backup drive (any external HDD that has a Time Machine backup on it) because it’ll be useful for backups on the new machine. It’ll even see that it’s a new machine that has all the same data and ask if you want to keep using the same Time Machine backup; this is call inheriting.
Alternatively, you can put the source machine into Target Disk Mode by pressing the T key when the machine is booting and having it connected to the destination machine through Thunderbolt or FireWire. Target disk mode only works through Thunderbolt and FireWire, don’t bother trying anything else. I’ve wasted enough time for us all: it’s not supported. If you’re transferring data from a machine with FireWire but no Thunderbolt to a new Mac that only has Thunderbolt you can get a Thunderbolt to Firewire adapter, but that’ll run you $30 and you might not have another use for it after the data migration. You could also use a Thunderbolt cable, but that’ll also run you at least $30, and again, you might not have another use for it, that’s why I recommend an external HDD. If you’re not doing a backup, it’s worth the peace of mind, and simplifies data transfers.