Confusion over the difference between DDR and DIMM probably results from a misunderstanding of the two terms, and some related concepts, such as RAM, DIMM and DDR. Put briefly, DDR is a type of RAM that is stored on a DIMM, though that statement may not make much sense if you're not sure what the terms mean.
RAM, or Random Access Memory, is a type of memory used by your computer for short-term data storage that allow stored data to be accessed in any order. This randomness is where the name comes from. Like other types of flash memory, it is a volatile form of data storage, meaning that it requires a constant power supply to store its data. RAM has much faster access times than nonvolatile memory such as hard drives, and it stores programs and files that are in use.
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DIMM stands for Dual Inline Memory Module, and it is the physical part of a computer that the RAM is in. DIMM is long, narrow, thin circuit boards with tabs along one edge; the tabs are studded with flat metal pins that transfer data between the RAM and the computer. The RAM itself is the black, rectangular modules on the surface of the DIMM.
Types of RAM
Different types of RAM come on different types of DIMM. Older DIMMs generally have fewer pins than newer types. The first DIMM was called SO-DIMM and had 72 pins, whereas DDR3 RAM has 240. DIMMs with different numbers of pins are incompatible with each other and cannot be installed in computers that are not designed for that specific type of RAM.
DDR RAM stands for "Double Data Rate Random Access Memory," which comes from the fact that it is twice as fast as the type of RAM it replaced. It was first released in June 2000, and was replaced by its successor DDR2 RAM in the spring of 2003.
Technically, there is no difference between DDR and DIMM, because DDR RAM is a type of DIMM. The difference between DDR and DIMM is something like the difference between a Ford Focus and a car. For example, if you are purchasing more RAM for your computer, you could buy a DIMM of DDR RAM. You could also buy a DIMM of DDR2 RAM or SDR RAM.