"Three Chasers try and score with the Quaffle; the Keeper guards the goalposts; the Beaters keep the Bludgers away from their team...."
-- Harry Potter (PS10)
"....every goal has been hard won against Beaters who on both sides are showing superb precision and power"
-- Ginny Weasley, Daily Prophet, match report Chad v Lichtenstein, 17 May 2014 (Pm)
Originally, the Bludgers (or, aptly, Blooders) hit by the Beaters were flying rocks, which could be cracked by the Beaters’ bats. This often meant that the resulting gravel would then follow the players around the pitch (QA6).
Beaters have to be strong and have a good sense of balance, as it takes strength to hit the Bludger very far and this sometimes needs two hands (so balance is a must). Beaters are more often Wizards than Witches because of needing this extra force, although Gwenog Jones is, as usual, an exception to this (QA6).
Two Beater moves in the modern Quidditch game are the Bludger Backbeat, where the Beater uses a difficult backhand swing, and the Dopplebeater Defence, where two Beaters combine for extra power (QA10).
Fred and George Weasley – Gryffindor Quidditch team
A beater can be someone who hits something or someone very hard - this is what the Beaters are doing to the Bludgers. A beater can also be a person who manages birds for shooting (Oxford English Dictionary). The British tradition of "beating the bounds", where the boundaries of a parish are defined once a year by going round them, is a much gentler affair (Wikipedia).
The story goes that the Wimbourne Wasps team got their name from one of their Beaters hitting a Bludger into a wasps' nest, stinging the Appleby Arrows' Seeker so badly that he had to stop playing, which led to a Wasps win (QA7).
Although the dates printed on the Daily Prophet Newsletters are:
the timeframe for these events is 1992-1993.
Lexicon list of Beaters
From the Web
Writing by J.K. Rowling on Pottermore:
Pottermore features: Quidditch
Harry Potter Wiki: Beater