GURPS Combat can be pretty "detailed", but is fairly simple at the core. Thus, this little "cheat sheet" is here to provide details on the manuevers and options available in combat. In general, only an outline of what options are available will be presented, to avoid reproducing the entire combat system.
The Combat System is based on 1 second turns, with each combatant acting in order of their Basic Speed rating (including any fractions). Someone with a Speed of 5.75 thus acts before someone who has a 5.5 Speed rating. In the case of ties, the person with the higher DX goes first or a single die is rolled to break the tie if DX is also the same.
A character chooses his manuever when his turn comes up, and then actually performs that manuever during the 1-second interval between his declaration and his next turn.
A manuever consists of some kind of action, combined with movement and defense options. Movement is generally restricted to a 1 yard step, which can be taken before or after ones action. Some manuevers allow more movement. Active defenses include Dodge, Block (with a Shield or Cloak), and Parry (with a Weapon, or Unarmed). Some manuevers and situations prohibit the use of some kinds of defenses.
In every turn, a character has the following options:
Free Actions: A character can undertake any number of these actions every turn.
Damage & Hit Points
There are two basic strength-based types of damage, "thrusting" and "swinging". Swinging is used when you are able to benefit from a lever, while thrusting is more "basic". With muscle powered weapons, this "Basic" damage is modified by the weapon itself, both in terms of a numeric bonus and in applying a damage type (see below) to the damage. Crossbows have their own ST value, and Firearms ignore strength entirely and inflict arbitrary damage.
Any damage you inflict is subtracted from the opponents DR, then multiplied as appropriate for its type and any hit locations you might have targetted.
Anytype you take damage, you suffer a Shock Penality on your next turn equal to the amount of damage you took. This maxes out at -4, and subtracts from your DX and IQ, as well as all skills based upon those attributes. It does NOT apply to any active defenses. Any wound that inflicts more than half your HP, or any crippling wound, is a "major wound". These wounds require a HT roll to resist stunning and being knocked to the ground.
A character reduced to below 1/3rd of his Hit Points is in "bad shape", and halves his Move and Dodge. Once you've been reduced to 0 HP or below, you have to roll versus your HT every turn in order to remain conscious. Once you reach -1xHP damage, these rolls are at a penality (-1 for every full multiple of HP below 0). You also have to make HT rolls to avoid dying for every full multiple of your HP you reach below zero. I.e., a character with 10 HP who reaches -10 damage has to make a HT roll to avoid dying and gets a -1 to stay conscious even if he lives through it. He makes another roll to avoid death at -20, -30, and -40 H Ps?. Once you reach -5xHP, you immediately die with no roll. Your body is utterly destroyed (if appropriate for the damage type) at -10xHP.
Here's the "wound modifiers" for different types of damage.
This damage modifier is applied after DR is subtracted from damage, affecting the inflicted damage.
You can choose to target individual hit locations...
You can attack at -8 (for the torso) or -10 (for anywhere else), in order to halve DR. This is cumulative with other armor divisors that you might have access to.
Deceptive Attack and Rapid Strike
You can make a "sneaky" attack, taking a -2 penality to your skill for every -1 penality to your enemies active defense. This only works with Melee attacks.
You can also choose to attack twice, each attack at -6. If you already have more than one attack for whatever reason, you can replace one of them with two attacks at -6.
I'll fill this in later, when I understand it better...
A slam attack lets you move your full Move speed and ram into an opponent, with both you and your opponent taking damage based on velocity and size (hit points). One person might get knocked down by this attack, with the attacker having a better chance of not being knocked down. If your opponent dodges your attack you must continue moving at least two yards past him if you can, possibly running into something.
Rate of Fire & Rapid Fire
All ranged attacks have a Ro F?, and if this number is over 1 that weapon can be used with Rapid Fire, in which case you can fire a number of "bullets" equal to the Ro F?. This gives a bonus to hit, and you inflict an extra "hit" dependant on the weapons recoil value and your margin of success. If your weapon has a Recoil of 2, and you succeed by 6 points, you scored 4 hits. Your number of successfull hits cannot exceed the number of bullets you actually fired.
You can Dodge most attacks, and have no limit in how many dodges you can make in a turn. You can choose to Retreat when dodging a melee attack once per turn, gaining a +3 to your defense. You may choose to Drop when dodging a ranged attack once per turn, gaining a +3 to your defense and moving away 1 yard. You can also make a Sacrifical Dodge, throwing yourself infront of an attack that would hit someone else.
Once per turn you can take an Acrobatic dodge if you have the Acrobatics skill, gaining a +2 to your Dodge if you make the roll and a -2 if you fail.
You can make ONE block defense per scene if you have a ready cloak or shield. These items also add a Defense bonus to all of your active defenses. You can choose to retreat when making a block roll, adding +1 to the defense roll and moving 1 yard. You cannot block firearms or beam weapons.
You can make one parry defense without penality per hand, either with a weapon or barehanded. Additional parries with the same hand take a -4 penality. Off-handed parries subtract -4 from the skill being used, and thus -2 to the parry roll. You can choose to retreat when parrying a melee attack once per turn, gaining a +1 to your defense and moving away one yard. Boxing, Judo, Karate and any Fencing skill give a +3 to your defense if you retreat. You can only parry melee attacks and thrown weapons (at a penality), unless you have special skills.
When you successfully parry an unarmed attack with a weapon, you can make an immediate attack roll to see if you damage the attacking limb. This is at -4 if the attacker used Judo or Karate. It's also difficult and potentially dangerous to parry weapons unarmed. You suffer a -3 to parry weapons unless its a thrusting weapon or your using Judo or Karate. A failed parry against a weapon can choose to hit either the original target or the arm you parried with.