• DRAM (Dynamic Random Access Module), 
  which are inexpensive and used for the computer's main memory
  
  
• SRAM memories (Static Random Access Module), 
  fast and costly, used for cache memory

DDR-SDRAM

The DDR-SDRAM (Double Data Rate SDRAM)
is a memory, based on the SDRAM technology.
It doubles the transfer rate of the SDRAM using the same frequency.


SDR - Single Data Rate

Data are read or written into memory based on a clock. 
Standard DRAM memories use a method known as SDR (Single Data Rate) i
it reads or writes a piece of data at each leading edge.

DDR doubles the frequency of reading/writing, with a clock at the same frequency, 
by sending data to each leading edge and to each trailing edge.

DDR2-SDRAM
achieves twise speeds of DDR at same external frequency.

QDR (Quadruple Data Rate or quad-pumped) uses two separate channels for reading and writing,

 
 Memory Speed depend on 

Desktops and server memory can be different

Synchronization (timings)
Number of clock cycles needed to access a piece of data stored in the RAM.

Latency : number of clock cycles that elapse between the reading command being sent and the piece of data actually arriving. 
In other words, it is the time needed to access a column.

Error correction
Some memories have mechanisms for correcting errors to ensure the integrity of the data they contain.

Parity bit
Modules with parity bit ensure that the data contained in the memory are the ones required. To achieve this, one of the bits from each octet stored in the memory is used to store the sum of the data bits. 

Dual Channel
Some memory controllers offer a dual channel for the memory.