All disks have bad block files on them as you can't manufacture a perfect disc (SSAs are probably different)
Running a verification pass on the disk using whetever diagnostic software causes the data to be read from each sector, test info written to the sector and then read back and compared. If the written and read data is the same it'll re-write the original data back to the sector. If the compare fails, it'll write the data to a spare sector on the disk which lies in a separate protected area of the disk, it then writes the a pointer from the failing address to the re-vectored address in the bad block file, so the data is accessed at its new location next time it's read.
The occasional bad block or sector shouldn't be worried about as the disk hardware is made to carry out this function transparently; The OS doesn't need to know.
However... If you start to see an increase in bad blocks being reported, then you should consider replacing the disk. You can have thousands of bad blocks re-vectored dependant on the size of the protected area on the disk (Usually two alternate cylinders so it depends on the No of heads, sectors, block size, etc)
If you're concerned, backup your data by all means, but don't go buying a new 1Tb disk because it's had one block or sector replaced!