SIMM (Single In-Line Memory Module) and DIMM (Dual In-Line Memory Module) are both types of memory modules used in computers. The main difference between them lies in their design and technology. SIMMs typically have a 32-bit data path and are used in older systems, while DIMMs have a 64-bit data path and are used in modern systems. SIMMs have a single row of pins, whereas DIMMs have pins on both sides of the module. Additionally, SIMMs are available in smaller capacities compared to DIMMs.