ECC is usually performed only on complete words, rather than individual bytes. In a typical ECC system with a 64-bit data word, there would be 7 ECC bits. Each ECC bit is calculated as the parity of a different subset of the data bits. The key to the power of ECC is that each data bit contributes to more than one ECC bit. The recomputed ECC is compared to the ECC bits read from the memory.
Any discrepancy indicates an error.
By looking at which ECC bits don't match, it is possible to identify which data or ECC bit is in error, or whether a double-bit error occurred. In practice this comparison is done by an exclusive-or of the read and recomputed ECC bits. The result of this exclusive-or is called the syndrome. If the syndrome is zero, no error occurred. If the syndrome is non-zero, it can be used to index a table to determine which bits are in error, or that the error is uncorrectable.