SD Ram is very large memory that is row and column based. It is great for burst access as it takes a while to set up into a new row, but can burst very quickly along the row. SD Ram tends to be cheap and use little power.
SRAM is great for random access, but tends to be slower and much smaller than SD Ram. it also uses alot more power and more expensive.
Flash is great for reading, but very slow at writing. Its usually used for storing boot up configurations that will be loaded repeatedly, but not changed very often.
