They fill parts of the rom with 00s or random bytes. You test every time until you change the data your looking for. Corrupters are obsolete if you know ASM in my opinion.

Corrupters go through a file and replace specific (or a range of) values to either a new specific value (useful) or a random garbage value (not as useful).  You can use them for when you're sure the desired data exists somewhere but can't figure out where.

Other corrupters also have a backup feature which will easily allow you to restore your ROM to how it was before you corrupted -- allowing you to corrupt a new range to narrow your search... or just restore the file so you can stop looking.


Example:  In a game the player starts with 8 life, and you want to change it so they start with 32

1)  Open a corrupter, tell it to search the ROM for $08 and replace them all with $20
2)  Run the rom.. see what changed:
  a)  If the game crashed, narrow your search to a smaller section (or just try a different section)
  b)  If the game runs, but the desired info didn't change, mark the section you corrupted as "not here" and try corrupting a new section
  c)  If the game runs and the desired info is changed, begin to break down that section until you find the specific value you need to change


Corrupters can also be used in other fashions, such as corrupting full chunks of the ROM to see what changed (to give you an idea of what information lies in that area)


As has been said... if you know how to trace, corrupting isn't really necessary, since you can usually just trace to find the info you want.  Corruption is still sometimes much faster and easier, though.... other times it isn't.