The term ECC refers to error correction as described in lengthy detail at http://members.aol.com/mnecctek/faqs.html#what Similar to Parity memory the installed corrects every single bit error seen in ram. Non-Parity or non ECC memory runs at a much faster speed since errors are corected by other means in newer systems. The main differences are outlined in the article seen at http://www.pctechguide.com/14Memory_Parity_memory.htm