How DRAM's are different from SRAM's?  Why DRAMs are said to employ address multiplexing?

Dynamic RAM (DRAM) is fundamentally similar as SRAM, except that this retains data for only 2 or 4 ms on an internal capacitor. In this after 2 or 4 ms, the contents of the DRAM should be fully rewritten (refreshed) because the capacitors, that store logic 1 or logic 0, lose their charges. The whole content of the memory is refreshed along with 256 reads in a 2-to-4 ms interval. Refreshing also arises throughout a write, a read or throughout a special refresh cycle.