Unlike high bandwidth memory (HBM, HBM2), GDDR5X is an extension of the GDDR5 standard that video cards have used for nearly seven years. Like HBM, it should dramatically improve memory bandwidth.
GDDR5X accomplishes this in two separate ways. First, it moves from a DDR bus to a QDR bus. The diagram below shows the differences between an SDR (single data rate), DDR (double data rate) and QDR (quad data rate) bus.\