Someone able to hold only one book at a time is going to be far slower than another person capable of holding a stack of eight at a time or, with high-bandwidth memory (HBM) in mind, 1,024.

Currently the most popular format seems to be DDR3 (Double Data Rate 3) and this is giving way to faster DDR4.

With the rise of graphics and GPUs, the GPUs are separate from a host CPU and memory, think compute RAM, and need their own memory, video RAM.

So standards came into being to evolve DDR3 RAM technology into graphics memory and GDDR4 and GDDR5 versions of that are based on DDR3 RAM.

A GDDR6 version is being worked on, by suppliers such as Rambus, with a DDR4 starting point.

With speeds up to 16 Gbit/s per pin, the Rambus GDDR6 PHY will offer a maximum bandwidth of up to 64 GB/s.“

The text above is a summary, you can read full article here.