ECC stands for ERROR CORRECTING CODE.
This uses technology on the motherboard to test the accuracy of of outgoing and incoming data by using a checksum. Some errors are automatically corrected, ECC modules are normaly used in high end workstations and servers where data integrity is vital. ECC applies to DDR, DDR2 and DDR3 modules.
ECC modules have a extra memory chip for every eight chips. eg 9 or 18 as opposed to 8 or 16 for a NON-ECC module.
In most cases you can install a ECC module into a NON-ECC system but generally a ECC system will require a ECC module. Please check your PC manual.