DDR

Double data rate DRAM means data transfer rate is doubled to the earliest RAM. The transfer happens at clock edges. 

Synchronous DRAM refers that the DRAM chip is synchronised zed with the memory controller through a shared clock. 

To increase transfer throughput, DDR4 device the memory into banks and grouping banks.  Transfer can be done simultaneously with multiple banks. 

Some systems implement multiple io controller that owns a set of bank to further increase throughput. Each controller is a channel. OS must be smart enough to layout the memory crosses the topology to maximize throughput. If all data is place in one bank, throughput is still serial. .