Anyone who has ever gotten serious about playing wargames will have come across the term powergamer. Like pornography, opinion rages over exactly what powergaming is, but everyone recognises it when they see it.
I'd say that a powergamer was someone who played the rules and not the game.
A powergamer might argue that his Eldar jetbikes count as skimmers and can only be hit in combat on sixes (even though he knows this is a stretch) while he will argue that your Khorne Berzekers can be pinned because their fearless description does not specifically state they are immune to pinning.
A powergamer will let you target his stealth suits THEN tell you that you have to roll nightfight to see them and THEN tell you that you could have shot the battlesuits behind because the stealthsuits don't block LOS. When it was bleedin' obvious you wanted to shoot the battlesuits in the first place.
A powergamer will place half of his vehicle off the edge of the board in order to get in a shot at your juiciest target which you have carefully shielded. When queried he would rather spend the next 30 minutes arguing his case than getting on with the game.
A powergamer carries twelve versions of his armylist around with him. He finds out his opponent's army and then uses the list best suited to killing that army.
A powergamer avoids playing armies and opponents who might threaten his precious 134-1 win record (and that lost game was against a - wait for it - POWERGAMER!).
A powergamer can usually be identified in your gaming group by studying his opponents. They will usually have their head in their hands by turn 2. They will be so browbeaten that by turn 3 they will let the powergamer get away with the most outrageous rules bending just to finish the game. The opponent will quietly weep after the battle at the inhumanity of it all. He is twelve and it his second ever game of 40k. The powergamer is 36.
Sometimes people use the term beardy instead of powergaming, but I think the two are different. Beards originate from historical gaming groups who usually have an older member (with a beard of course) who knows everything about everything and CAN use his knowledge to influence the game. A powergamer WILL use every tool at his disposal to win the game.
The powergamer will carefully choose his opponent and the army he is to fight, he will optimise his armylist to fight that foe without giving his opponent the same courtesy, and he will use every rules loophole, codex mistake and arguing technique to force a win, even against new players.
That's why powergamers and unsporting players are often described as one and the same.
It's just not cricket!
You know you are a powergamer if the units and rules you rely on to win games are changed when a new edition of 40k comes out or your codex is updated.
2nd edition powergamers, for example, had to go out and buy tons of troops in order to make their armies legal for 3rd edition. I'd imagine that a lot of current players with 5+ rhinos in their marine armies won't be using as many as often in 4th edition.
Powergamers tend to gravitate toward the powerful armies and the powerful rules within those armies. Although the game designers try to balance the game it is so vast, wide ranging and sprawling that they will never get it right all the time. Eventually the problem is identified and rectified and then the power gamer has to move on to another rule/army.
'Regular' gamers won't focus quite so much on rules; they have other considerations like aesthetics (how the models look), theme (the idea behind the army), originality (nobody has an army like mine).
For myself (and I suspect most gamers) I shift between the different styles of gaming depending on my mood. Although I'm predominantly an aesthetics player (I only have a chaos army, for example, because the daemon prince model is so cool) and I'm often faced with the dilemma of crafting a bunch of good looking models into a decent army, there are times I want to win and I deliberately pick my 'hard' units.
In those games I am more of a powergamer because I'm playing the rules and not the game.