SO DIMM is Small Outline – Dual In-line Memory Module.  These modules are generally used on laptops and notebooks and they are relatively smaller and thinner compared to those used on desktops and servers.  A typical 32-bit EDO or FPM SODIMM has 72 pins. More commonly used today are the 64-bit SDRAM SODIMMs which have 144 pins for SDR and 200 pins for DDR.

Still smaller than an SODIMM is a MicroDIMM. This type of module is used in sub-notebooks and has 144 or 172 pins.