Fully Buffered DIMMs are used in servers to increase reliability and to access more memory within a system.
Fully Buffered DIMMs (FB-DIMM) have an AMB (Advanced Memory Buffer) unlike other buffers this has a Serial Interface to the memory controller, this allows greater data width without having to increase the number of pins on the memory controller. This method is only used in DDR2 modules.
FB-DIMMS have 240 pins and are the same length as DDR DIMMS but differ by having notches in different positions so standard DDR2 modules will not fit into a FB-DIMM system. FB-DIMMs require a FB-DIMM compatible system.