Registered vs. unregistered (unbuffered)
Memory modules can be built with additional support chips to buffer the flow of data on and off the module. This provides less load to the motherboard, increases reliability, and allows for the use of larger or more memory modules. There is sometimes a small speed penalty to this. Because of the added expense of the modules and the performance hit, the use of buffered or registered memory is confined mainly to servers and high end workstations (and some early Athlon 64 motherboards) If a machine requires registered memory (or unregistered memory) do not install RAM of the opposite type. Even if it supports both, never mix registered and unregistered modules in the same machine.
Unless specified otherwise, all standard RAM is Non-ECC, unbuffered and unregistered.
SDRAM memory modules have 168 pins and two notches (Wikipedia article).
DDR memory modules have 184 pins and one notch (Wikipedia article).
SDRAM modules have 240 pins and one notch. The notch on DDR2 DIMMs is in a different position than DDR DIMMs, and the pin density is slightly higher (Wikipedia article).