Memory Type: DDR3 PC3-8500, DDR3 (non-ECC)
Maximum Memory: 16GB
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.
Install Guide for your:
iMac 2.66GHz Quad-Core Intel Core i5 (27-inch - DDR3) Late 2009
video - installing memory