What's the difference between buffered and unbuffered DIMMs?

High density DIMMs have lots of chips on them and therefore possess a higher capacitive load on the address and control signals in comparison to lower density DIMMs. Some designers use redrive buffers on the DIMM to boost the signals to reduce system loading when compared to the same high density module without buffers. But the buffers introduce a small delay into the electrical signal, so adding buffers to a standard density module would have the effect of slowing down the signal, compared to the same low density module without buffers.