Relationships and differences among kinds of memories ?

DRAM ( dynamic RAM ) are cell's that holds data using capacitor, so they are usually slower than other kinds, but they're cheap (~1 trans). an example of DRAM is your computers RAM module. problem is, the capacitor will discharge, so they have to rewrite time to times ( using onboard dram controller ) to "refresh" data.

SRAM ( static RAM ) on the other hand are flip-flip based, they catch a bit in their circullar path. they are small, and fast but need ~6-8 trans. many of the onchip memories ( like CPU's cache, rf, scratchpad ) are SRAM based.

I think Altera's Ram block like m9k follows this too.

SDRAM is DRAM with Sync. clock interface. like computer's RAM module.