Right by now you should deep in study and the difference between SRAM and DRAM may be giving you some grief. Have a look at this article.

Basically, SRAM is fast and expensive and DRAM is less expensive and slower. Now, there is a lot more to it than that so get out your notes and go look it up. P46 of the Scholar notes has this to say

SRAM chips are very fast but are not suited for very large amounts of memory. They are more suited to cache memory, where only small amounts are required. You will learn more about cache memory when we look at factors that affect system performance.

DRAM chips are more widely used. They are much cheaper to produce, can hold larger amounts of data in a smaller physical area and require less power. They are dynamic, requiring a continuous signal to refresh the contents of the chip.