A downvoter is someone who uses a YTMND account mostly to downgrade the rankings of many sites by giving them the lowest possible score. This is known at YTMND as downvoting. Once a site has been downvoted, its overall rating suffers, making it difficult to restore and less likely to draw attention to potential viewers. What sets a downvoter apart from a normal user is how a downvoter almost always rates each site with ,which is the lowest vote one can get, and compromises 50% of the sites votes(the other half are 5 stars). They do this regardless of how good the site is.
Downvoters often perform their actions by going to the profile of a targeted user and downvoting all their sites from there. This way, they don't even have to look at the user's sites and can downvote easily en masse. Max has taken measures to prevent or slow down downvoters by deleting their accounts. At one time, Max deleted 2,211 downvoter accounts, reversing over 250,000 votes.  However, some of the more famous downvoters (such as Adverb, inkdrinker, and Tehpwner2) remain on the user list and are active in their downvoting.
The opposite of a downvoter is an upvoter.