Apple is refreshing the iMac line-up, with new specs and displays. All the 27" model now have a 5K compaired to only one model. That means that the starting price for a iMac with a 5K display now starts at $1,799, compaired to the old price $1,999. Apple is also stating that the new display can output 25% more colours.
Inside the iMac, the processors are now Skylake CPUs starting with a 3,2 Ghz i5, and up to a 4,0 Ghz i7. The graphics are now handled by new AMD GPUs starting with a 2GB M380 and up to a 4GB M296X. RAM has seen a minor bump up now with 8GB 1867MHz going up to 32GB. It now also features a Thunderbolt 2 port.
The 21,5" iMac also got an upgrade, the highest end model ($1,499) now has a 4K display, with the 1080p models starting at $1,099.
As stated earlier, the new 4K display has the same new display tech as the 5K iMac. Inside the 21,5" iMac there's in the cheapest model a 1,6GHz dual-core i5, and the 4K model has a 3,1GHz quad-core i5 with Iris Pro graphics. The RAM now is 8GB of 1867GHz LPDDR3 going up to 16GB. Just like the 27" the 21,5" now has two Thunderbolt 2 ports and also more storage options
They're also new versions of the keyboard, mouse and trackpad. They've now got rechargeable batteriers charging over the Lightning cable. The keyboard is much thinner because of this and the trackpad is bigger, but the Magic Mouse is the same as before. The new keyboard and mouse will come included with the new iMacs.
The new 27" and 21,5" iMacs are all shipping today.
Sources:
http://www.theverge.com/2015/10/13/9512503/apple-imac-update-27-inch-5k-price-release-date-specs
http://www.theverge.com/2015/10/13/9512505/new-apple-imac-announced-specs-price-release-date-21-inch
http://linustechtips.com/main/topic/466952-apple-releases-215-4k-retina-imac/
Edit: Added info about the 21,5" and added more images