The iMac "Core i5" 2.9 21.5-Inch Aluminum (Late 2013/Haswell) initially featured
a 22 nm "Haswell" Quad Core 2.9 GHz Intel "Core i5" (4570S) processor with 
four independent processor cores on a single chip, 
a 6 MB shared level 3 cache, 
8 GB of 1600 MHz DDR3 SDRAM,
a 1TB Hard Drive, and 
a NVIDIA GeForce GT 750M graphics processor with 
1 GB of dedicated GDDR5 memory. 

It also has a built-in "FaceTime HD" webcam and stereo speakers, but lacks an optical drive.

This model has a 21.5" 1920x1080 LED-backlit 16:9 widescreen IPS display with the "cover glass fully laminated to the LCD and anti-reflective coating." 

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 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 much more efficient "Fourth Generation" processor and architecture, as well as faster graphics, faster Wi-Fi, and the option of faster storage.

Please note that stock images are used