Time for another exciting installment of the thing everyone likes the most: grammar lectures on the internet!
...Where is everybody going? What's wrong? Oh god what have I done. No, um, it's not a grammar lecture, it's er, a tactics article! Yeah, that's right! I'm gonna teach you important things about playing the game and winning and stuff!
(Oh god I hope that sounds believable...)
Rock: A type of army, also sometimes known as "hammer" or "deathstar." Rock armies get the spotlight early on like this because they are an easy mistake to make- the are the epitome of "all your eggs in one basket." A rock army, simply put, is one that has a single plan, virtually always revolving around delivering some sort of strong assault unit into the enemy's lines. If that plan fails, it has no other options, no real backup strategy, just one six-hundred point investment and some hope. While this can certainly win you fights, it has several inherent problems, not the least of which is you have no options. Guard blob army? Rush forward and assault with your rock. Blood Angels Doa list? Rush forward and assault with your rock as soon as they're on the table. Mech Eldar mobility list? Rush forward and assault with your rock. You have only one real plan and thus have no way to adapt to your opponent's weaknesses.
On top of that, there will be times when you face a "natural counter" to your army- that is, an opposing army that seems custom-built to defeat you. A Land Raider rush will roll over and die to a Dark Eldar lance spam army; Genestealer Rush folds badly to Purifier Spam; etc. Non-rock armies will sometimes come up against a natural counter as well, but they have more options when this happens (since a rock army is, by definition, lacking in options) and are less likely to encounter such an army (since the more complex and layered your plan is, the lower the chances of finding an army perfectly suited to beat it.) On top of that, rock armies tend to be a bit unpredictable in performance, since all of their force is concentrated into a small number of units- it does not take very many poor rolls for them to collapse entirely; of course, this works both ways, as some good rolls can make them nigh-invulnerable, although against a good opponent this still may not win you the game.
For these reasons, taking a rock/deathstar/hammer army to a tournament is a risky proposition at best. 'Ard Boyz and similar tournaments that greatly reward massacres or other strong finishes will show an undue percentage of rocks in the top spots, however, since "merely" getting a perfect record is no guarantee at all for getting first place at such an event.
As with most of the terms here, there are degrees to rock armies; some may have a limited backup plan, or might only contain a small rock- that is, have one "big" unit to threaten the enemy with whilst trying to preserve their normal battle plan. While such tactics are, more often than not, somewhat futile, as making a rock army work to full performance tends to demand all the resources in a list, but lists can certainly be said to have more or less "rock nature" to them indicating the degree to which they fit the stereotype.
Thunder Hammer/Storm Shield Terminators in a Land Raider (or, more commonly, two sets of them) are one of the more well-known rock armies; others include Nob Bikerz, Tyrantstars, Stormravens+(whatever), Jetseer Councils, and the so-called Leafblower. Most of them are characterized by one, or sometimes two identical, powerful melee units with extreme resilience and the ability to tie up or destroy large parts of the enemy army as well as the ability to close and engage quickly. The shooty variants, such as Leafblower, tend to try and alpha strike the opponent out and usually have less resilience than their melee cousins, though this is mostly because the melee versions need it more.
Balanced: The antithesis of a rock army, a balanced army is one that does its best to cover all its bases and be prepared for every contingency. Of course, it will still have a primary plan, but this tends to be much more generalized than with rock armies and will have redundancy and options in the extreme so as to be able to counter whatever threat the enemy puts forth against it. A balanced army will usually feature a wide variety of weapons across different platforms, or at least as wide as its codex permits and makes feasible, and will likewise generally have both melee and ranged defensive and offensive options.
The advantage of a balanced army should be pretty obvious- no matter what the situation, you theoretically have a tool to deal with it. The disadvantage, however, is similarly glaring- your army is going to have less of any single thing that a more focused army might. In a well-designed army this is not so much of a problem, as you should be taking a sufficient number of each item that they aren't easily eliminated and even non-ideal units can still perform secondary roles (such as a Hydra putting wounds on monstrous creatures, or Meltaguns killing MEQs.) The trick, of course, is getting that balance right, which can involve lots of tweaks to the army and a willingness to abandon setups that can't be shaped into the necessary form, not to mention the skill required to assess the army and its needs accurately and synthesize them into a functional whole.
You'll find Kirby (and others) strongly espousing the virtues of a balanced army, but I enjoy playing devil's advocate here, as everywhere else; there are potential advantages to having a rock-ish army, or at least the ability to play like one, and I believe there is nothing wrong with taking your local factors into consideration nor to intentionally leaning towards one particular spectrum of capability if one knows what one is doing.
It is worth special mention here that a balanced army does NOT try and spam guns (like Melta) because they are nominally "good"- this is the opposite of what a balanced army is trying for. While some players might find their sensibilities offended by identical squads (which is entirely within their rights, though it confuses me to no end), a closer look will note that other weapons will be liberally mixed in as well- for example, a Guard list might feature several squads with triple or quad Melta, but will also have ones with Autocannons, Lascannons, Multilasers, Flamers, etc, all in non-ignorable quantities. The exact numbers will depend on the relative value of each gun and its utility to the army, so it's only natural that some are more prevalent than others, but simply including large numbers of the "best" gun is not the way balanced armies function.
MEQ: MArine Equivalent; a model with T4 and a 3+ armor save, and the rest of the standard Space Marine statline, although the toughness and armor save are the most relevant parts, since it's most commonly used when comparing the effectiveness of various weapons at killing particular targets. The term is important because Space Marines are the "golden standard" of targets in the game, as you will fight them more often than nearly anything else and the profile is common even in non-Marine armies such as Necrons and even Tyranids. MEQs are the general "tough infantry" template.
GEQ: GUardsman Equivalent; a model with T3 and a 5+ armor save; as with MEQ, the assumption of the rest of the statline is also sometimes used. Also like MEQ, the term is used mainly when talking about the ability of various weapons to efficiently kill certain targets- in this case, GEQs are the default "easy target" and tend to represent the variety of minimally-defended horde armies out there. With the advent of 5th Edition shooting results almost always calculate in their 4+ cover save, as it's generally a safe assumption that their owner is a smart man and stuck them somewhere safe.
TEQ: TErminator Equivalent; a model with T4 and a 2+ armor save. Much less commonly used than the above, since with many weapons it will result in identical numbers to the MEQ statline, but against guns that rely on rate-of-fire the difference is relevant because Terminators will only fail half as many armor saves as normal Space Marines. As with the other two, the assumption of the standard gear/stats is not uncommon (i.e. Storm Bolter + Power Fist), especially when referencing roles for certain units in an army, as in "I used Termagants to deal with TEQs most of the time." A less common version of the term is "Tau EQuivalent," meaning a T3/4+ model, but this is mostly out of favor in 5E thanks to the relative rarity of such models (only Tau and upgraded Guardsmen) and the general assumption that all models will have a 4+ cover save when being shot at.
