The main difference between DIMM and RDIMM is that DIMMs are used in regular computers while RDIMMs are used in servers. DIMMs have a 64-bit data path, while RDIMMS have a 72-bit data path. This means that more data can be transferred per clock cycle on an RDIMM. Additionally, RDIMMs allow for error correction code (ECC) to be used, which helps to prevent data corruption.