I had it sharpened just today.
Now, I'm coming in there with my rules
That you must follow when you play."
A Scrub is a player of a competitive video game who adamantly believes that their "house rules" should apply to everyone to promote their view of "fair play". If a scrub sees a move or strategy they don't like (or can't beat), they ban it (if only in their own mind), and complains that anyone who uses it is cheap.
The video game is what it is. It defines what you can do and what you can't do. In competitive games, the rules exist as written into the game world. Sometimes, this leads to an over-powered character. Sometimes, it leads to a Game-Breaker.
Some competitive video game players accept that some characters/races/etc. are more powerful than others. When they're being competitive, they play one of the characters/races that are on the top tier and ignore the rest.
Some people do not hold to this. They develop house rules designed to bring what they feel is a game imbalance back into balance, so that there is more variety (after all, if a game has 20 characters, but you would clearly win only using two of them, then those 18 other characters are not worth playing). Essentially, a self-made game patch. This is all well and good. A classic gaming example of this is the ultra-common "No Oddjob Rule" in Goldeneye, often put in place because Oddjob is a Game-Breaker due to his size.
However, this self-limiting with house rules can go too far. When it does, you have the Scrub.
The mistake the Scrub often makes is making up rules too soon. The Metagame can often turn an apparent imbalance on its head. A lower tier character can become a higher tier one, or vice versa. Or something that seemed initially very unbalanced can be countered with time and effort at learning the tactic. Alternately, with the advent of patching, more modern titles may rebalance the game so that the former Game Breakers aren't as unbeatable as they used to be. The Scrub circumvents this by simply banning something without making a good faith effort in actually getting around it with the in-game rules.
The Scrub will often consider the tactics that he bans "Game Breakers". However, what constitutes a Game Breaker varies from person to person. Within a particular skill level, a game can be broken in that a simple repetitive tactic can lead to success. A more skilled player could stop it, but if it requires a great deal more skill to stop the tactic than to perform it, then it can easily appear to be a Game-Breaker. Not that it matters; even if the "offending" player changes to a different strategy and wins again, the new strategy is also "broken".
What ultimately makes the Scrub undesirable isn't the rulesetting; it is the attitude. Players will often agree to refrain from using certain moves or from selecting certain characters so they can have fun without investing a lot of time into learning the game. The Scrub, on the other hand, believes that his way is the only proper way to play the game and refuses to acknowledge that the game could be more fun any other way.
Scrubs may also put down others who use certain characters/teams/etc. for simply using them like everyone else is using in order to win. In other words, they don't like it when people use something that everyone else is since it's not "original" and will even go as far as accusing these people at sucking at the game for not using anyone or anything besides the top tiers. In Collectible Card Games, this is often referred to as "net decking" because they believe the player took their deck idea from a successful deck they found on the internet.
Compare and contrast with "Stop Having Fun" Guys, who are at least as annoying as the Scrub. The Scrub is the antithesis of the "Stop Having Fun" Guy, although one could argue that the two are very similar — "Stop Having Fun" Guys demand that everyone use the most efficient and powerful choices (with failure to do so considered "inferiority"), while Scrubs insist that everyone follow their personal house rules (with failure to do so considered "cheating"). Both make the mistake of taking things very seriously, both are more than willing to start a Flame War rather than let another kind of player exist in peace, and both are eager to argue with anyone who doesn't agree with their style of play.
Scrubs are often the result of Min-Maxing / Whoring. When a player sacrifices certain skills they deem useless in order to bolster skills they deem superior, they usually aren't happy when another player has a build meant to exploit their weaknesses. These scrubs will often argue that whatever mechanic they are choosing to ignore isn't part of "the real game," and that it should be removed.
Combined with the Noob, a scrub can quickly turn into a God Modder, who actually does cheat or abuses glitches to the extreme, but still considers his tactics to be fair. And then there's the "tryhard", a low-level player with a scrub-like mentality who fancies themselves a high-level player, and tries to adopt high-level tactics even though they have no idea how to actually pull them off. Also note that because all of these labels are "bad", most people do not consider themselves to be any of them—someone who fits the description of "Scrub" will just think they're normal while accusing any opposition of being "tryhards" and "SHFG"s, or any such combination thereof.
On the other hand, the term "scrub" can be used by players to indiscriminately insult players they deem unworthy, or simply players they don't like. The word "scrub" gets thrown around so much in the World of Warcraft community that it has lost its original meaning. (Although in reality, the meaning of "scrub" to simply mean "incompetent" came first, and using the term Scrub as a trope to define the mentality as David Sirlin did came later.)