Tying into my post on Broadsides vs. Frontal Assaults, here's one on the use of debuffs and buffs within a group setting. Mind, I'm not as familiar with these skills as I am with weapon performance/enhancement, so I may get some of it wrong, hence the "theory" part of the title.
A Buff is usually something that helps to keep yourself or a friendly ship in working order, usually by increasing damage resistance and increasing regeneration/repair over time, but also in that they can repair damaged subsystems such as shields or weapons.
A Debuff is something that lowers resists or disables an enemy ship in some fashion.
In practice, you use these abilities on opposing/friendly ships to weaken/strengthen them to make them easier/harder to kill. There's a wide variety of both sets, from all career paths for both Officers and Players, and there's quite a bit of counter/re-counter mixed in to keep things interesting.
Now, I know what you're thinking; "If I have support abilities, I won't be able to keep myself alive" and this is true in a sense, but worth considering is that no matter what buffs you have on your ship, your team mates stacking buffs on top of you will produce a far more powerful and redundant tank than anything you could ever manage on your own. This is only possible if you work together to achieve this though and you can only work together if you can properly support one another.
Good news is, pretty much every ship is capable of supporting a teammate, even a hardcore Tactical Captain and his Escort.
Even an Escort can be a tank if half of it's crew consists of Science and Engineering Teams.
On the flipside, you have debuffs, such as Attack Patterns Beta and Delta. These weaken an enemy target and I believe they also stack, even going so far as to result in negative resists. These abilities are strong in the sense that they throw off teamwork. Most times, people will react to concentrated enemy fire. They aren't quite as ready to notice debuffs being stacked on an ally while a lone Escort is bearing down on him, tactical abilities locked and loaded.
Also worth noting is that, I believe, that some resists debuffs result in shields being weakened across the board. Meaning that even their kinetic resists drop. Meaning that torpedoes will be doing full damage to shields. Reverse Shield Polarity isn't worth anything against that.
And, as a curiousity is the seemingly underrated Boarding Party/Security Team abilities. The question is; what happens to a ship that has been successfully boarded by 20 or 30 assault shuttles? Nothing good I wager. Viral Matrix that stacks and has a short cooldown maybe? Your team could take an enemy ship completely out of the fight for a period of time and do so again before anyone is the wiser. Naturally, it's best to let the boarding parties sort out the enemy ship before destroying it, so intiating a boarding against a high tier support ship would probably be best.
To be honest, I'm not sure how stacking is treated with either Buffs or Debuffs. Whether both stack, only one of them does, if there's a hardcap in there somewhere (negative resists?), etc. These would have to be answered by experimentation within a team setting.
Personally, I suspect that debuffs stack more strongly than buffs, but have a hardcap. This isn't from in game experience, but my experience with games and mmos in general.
Tying into the post above, here's a post on comboing abilities on an individual and group basis.
A combo is when you use two or more abilities to gain spectacular results. Much like using energy weapons to knock down a shield facing and then delievering a torpedo into the target, combos are using the proper sequence of powers all deployed with the correct timinng.
Being a tactical ability, these powers are frequently overlooked in favor of seemingly more reliable and powerful abilities such as High Yield or Beam Overload. However, they're also sometimes the only reliable abilities available. Polarize Hull will defeat a hundred Tractor Beams. Reverse Shield Polarity will defeat a 1000 energy weapons. Science Team will purge Viral Matrix, which itself takes time to deploy. All common and powerful support abilities.
Being beam attacks, the effects are instant without having to wait for a probe to crash into the enemy. This makes timing easy with practice. A team that successfully uses Target Shields and fires multiple full spreads at a group of enemies caught within a gravity well will achieve kills results like nothing else in the game.
Plus there's only a handiful of counters to Subsystems being disabled and Engineering/Tactical Teams aren't so good that opponents will always have them or that the crew subsystem won't be on global due to the use of Science Teams.
The most popular Subsystem to target is of course shields. The most useless if it doesn't completely disable the system, the most powerful if it does. If you're using AOE attacks, such as Torpedo Spread or Gravity Well, this will be your bread and butter ability. May work well especially well to break up Fed Ball.
Overlooked, but reliable in it's own way. The existence of Attack Pattern Omega, Evasive Manuevers, and Polarize Hull, make this a must have ability in making sure the enemy doesn't get away. All of those abilities become quite worthless if a target's lost all powers to engines.
My personal favorite, even if it doesn't completely knock an oppenent's weapons off-line, it's never a bad thing to temoporaliy weaken an oppenent's weapons. Meant to be spammed unlike the other abilities, imo.
Despite being seemingly useless, this may be useful as both a spammed ability to reduce the effectiveness of offensive and support abilities, while also being able to act as a strong counter against certain Engineering abilities that boosts a target's Hull Resists. Even more vital than Target Shields for breaking down tanks.
Jamming/Scramble Sensors, Boarding Parties, and High Yield Plasma
Previously mentioned, this is a strong combo both on an individual basis, but mainly within a group. A good opener when approaching an enemy fleet, as opposed to rushing head long while flying at Full Impulse. There are hard counters to these abilities being used en masse, such as Torpedo Spread, Tractor Repulsors, and Fire at Will, but I suspect that those abilities will rarely be used in a PVP setting, since people that fly Escorts are, contrary to their most likely occupation, not the most Tactically minded of people.
Viral Matrix and Everything There Is
One All Powerful Ability and one counter. Good for personal use and in a team setting. Means you won't be able to scramble enemy sensors if you use it though. You must consider it's very presence carefully, due to the strong temptation to have it for yourself.