The 2015 edition of the 21.5" iMac has some newfangled specs:

1.6 GHz dual‑core or 2.8GHz quad‑core Intel Core i5

8 GB (4x2 GB) of 1867 MHz LPDDR3 RAM (Configurable to 16GB)

Intel HD Graphics 6000 or Intel Iris Pro Graphics 6200

1 TB (5400-rpm) hard drive, configurable up to 1 TB Fusion Drive or 256 GB of flash storage.

802.11ac Wi-Fi + Bluetooth 4.0

More on those accessories later...