The newer Thin Series 21.5" iMacs use a 2.5" SATA drive. These newer systems are a bear to open so you do need to be very careful as you can kill the display very easily. Follow the IFIXIT guide here to the letter and make sure you use the correct tools: iMac Intel 21.5" EMC 2638 Hard Drive Replacement.
As to what drive to put in this gets tricky here as the larger SSD's are still quite expensive and Yosemite does not allow you to use TRIM Enabler due to the kext signing Apple is now using. You will need to get a SSD that does its own garbage collection onboard. Sadly most the SSD vendors don't tell you what they do internally on their web sites. We use Crucial but you do need to speak with their tech support to make sure the SSD you have selected is ideal for your system. Samsung also makes a good unit but you'll need a UNIX boot up disk to alter its settings which is a pain.
The other option here is is to go with a SSHD hybrid drive. Here a SSD is used as a deep cache for a standard HD. Basically you gain the boot up speed of the SSD and the deep storage of a traditional HD. I do recommend the Seagate SSHD's.
Frankly, If you have never opened a iMac before you should just think about going with an external RAID drive using Thunderbolt. These could even be SSD's or a mix of both, then alter your boot up setting to boot up off of the external drive to get the zip.
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