The essentials exam may ask you to identify ram and these can be difficult because ram looks alike. SD-RAMis can be identified because it has 2 notches along the pins. DDR and SIMM have 1 notch but simm has far fewer pins then ddr. SIMM is considered legacy its unlikely is will appear in the exam.
RDRAM usually has a metal cover that makes them easily identifiable and sodimms only have 4 black rectangles that make them stand out. It is highly unlikely youll be asked to identify between DDR and DDR2 but DDR2 has small black squares instead of long rectangles but this depends on the manufacturer. The notch of DDR2 is also different.