The iMac "Core i5" 3.1 21.5-Inch Aluminum (Retina 4K, Late 2015/Broadwell) features
a 14-nm "Broadwell" Quad Core 3.1 GHz Intel "Core i5" processor (5675R)
with four independent processor "cores" on a single chip,
a 4 MB shared level 3 cache,
16 GB of onboard 1867 MHz LPDDR3 SDRAM,
a 1 TB hard drive,
and an "integrated" Intel Iris Pro 6200 graphics processor that shares system memory.

It also has a built-in "FaceTime HD" webcam and stereo speakers.

This model has a 21.5" 4096x2304 LED-backlit 16:9 widescreen IPS DCI-P3 "Retina 4K" display (that runs "pixel doubled" to look like 2048x1152).

The rear of the case is aluminum and is thicker in the middle and tapers to a razor thin 5 mm at the edges.

Connectivity includes
four USB 3.0 ports,
dual Thunderbolt 2 ports,
Gigabit Ethernet,
802.11ac Wi-Fi (802.11a/b/g/n compatible),
and Bluetooth 4.0.

Compared to its predecessor, this model essentially has the same external enclosure, but it has a vastly higher quality and higher resolution "Retina" display as well as a more advanced architecture and graphics architecture.

Please note that stock images are used.