In short, 204-pin small outline dual inline memory modules (SODIMMs) are the form factor used to provide DDR3 memory modules for notebook computers and other small form factor computing platforms. 204-pin SODIMMs are currently available in PC3-8500 (DDR3 1066MHz) and PC3-10600 (DDR3 1333MHz) speeds.
To use DDR3 memory, your system must have both a 204-pin SODIMM slot and a chipset that supports DDR3 memory. A DDR3 SODIMM will not fit into DDR2 or DDR memory sockets. (Information about which memory technology your system uses is included in the Crucial Memory Advisor™ tool.) SODIMMs are smaller and thinner than regular DIMMs, and the 204-pin SODIMM PCBs are always ~2.6 inches long and are usually around 1.18 inches high (though the heights can vary slightly).
They consist of a number of memory components (usually black) that are attached to a printed circuit board (usually green). The number of memory components on a 204-pin SODIMM's printed circuit board (PCB) can vary, but the PCB will always have 102 pins on the front and the back both, for a total of 204 pins. The gold pins on the bottom of the SODIMM provide the connection between the memory module and the memory socket.