I was lying in bed yesterday thinking about things like this, so I'mma write it down. There will be sequels on the finer points, but anyway have a bit of basic RPG combat typology.
In RPGs generally actions are determined to succeed or not succeed by comparing two values of success. Usually, the conductor of an action has the ability to somehow influence his value, and be it only because randomization is involved; the value of the obstacle is generally passive and unchanging.
However, in combat situations, this is only one of the possibilities. Both values might be active, or indeed only the defence might be active, defending against a constant attack value. This article represents my musings on different ways of handling attack/defence in combat (possibly including social encounters/political debates in systems that use the same rules for both).
This post makes the following assumption that there is, maximally, only one step of randomization involved.
In many systems, both hitting and doing damage involve rolls. This is particularly egregious in D&D, where until very high levels the bulk of either is randomized. This leads to defence being highly overpowered if you go by the statistical mean, but also that an attacker with luck on their side can without problem absolutely devastate any defence (made especially bad by the critical hit rule that cases a roll of 20 on the attack die to hit automatically, ignoring the AC of the defender entirely). Such a clusterfuck is both terrible to balance and to make any sort of predictions about (and so luck-based that it arguably reduces the meaning of player strategy), so we will not deal with it.
Active Offence, Passive Defence:
There is no defence. The attacker determines their damage and subtracts it directly from enemy HP. This can also be a double-passive system in which damage, as well as HP, are constant and determined only by stats and equipment. Can turn strategic with the appropriate rules but may be too easy. Does have the advantage of being possibly the easiest system to balance.
The defence has an avoidance value compared against by the offence. If the offence fails, no damage is dealt. A more luck-based option. Alternatively, there can be multiple steps of avoidance; in this case, it acts more like variable damage reduction.
Hits always connect, but there are ways to reduce the damage. Damage reduction scales less well than pure Hit Points do, as it may invalidate weaker hits completely.
Avoidance and Damage Reduction
The offence must overcome both a barrier to hit at all and then have its damage reduced by the defending value. Makes for long play and is generally not recommended for systems that use hit points; can be an option for wound-based systems if realism or less lethality is a greater goal than speed of play.
Passive Offence, Active Defence:
Basically, some sort of action taken to reduce damage. This can be done with rolls or with an action point system; some particularly complication-happy systems combine them.
A combination of some of the above. Many active-defence systems are this; especially ones based on action points are by necessity. Roll-based systems have greater freedom, however.
That was all I could come up with today. Additions, corrections & complaints are welcome.