DDR3 PC3-10600, DDR3 PC3-12800

Memory Type: DDR3 PC3-10600, DDR3 PC3-12800, DDR3 (ECC)
Maximum Memory: 32GB
Slots: 4
Each memory slot can hold DDR3 PC3-10600, DDR3 PC3-12800 with a maximum of 12288MB per slot.*
*Not to exceed manufacturer supported memory.

You cannot mix Registered and Unbuffered memory. If you use the Crucial 16GB Kit (CT1431612) or 24GB Kit (CT1469658) all previous existing memory must be removed.

Memory or DRAM is the 'working' memory of the computer. It's used to store data for programs (sequences of instructions) on a temporary basis.

video - installing memory