Most laptops use SO-DIMM memory, while most desktop (or other full-size form factor) systems use DIMM memory sticks (A and C, in this case). Dual inline memory module sticks were brought into the market to alleviate the issue of having to use matched pairs of memory modules that SIMM (single inline) systems had before the Pentium 5-based motherboards. Small outline DIMM is simply the smaller version of the form factor used in laptops and other portable systems.
