The density, high or low, of a memory module refers to the number of chips that are on that module. Different computers require the memory module to be built with a different number of chips. i.e. There is more than one way to build a 1 GB memory module.
All low density 1 GB modules are made with 16 chips (8 chips on each side) using 64Mx8 device whilst high density 1 GB modules are made with 16 chips (8 chips on each side) using 128Mx4 device.
Low density modules have 100% compatibility with ALL systems and ALL chip sets as they fully follow JEDEC ( Joint Electron Device Engineering Council ) guidelines.
High density modules only have 10% compatibility ( mainly designed for used in slower server chip sets ) and are very slow as they break all JEDEC guidelines and are usually about half the price when compared with low density modules.
Hope this helps