During your turn you normally have the time/ability to perform two activities. You get two Actions, you can switch out an action for another maneuver if you're so inclined, or strain yourself to get an extra maneuver. But you can only use two maneuvers per round.
Action - Used for attacking, performing skill checks etc.
Maneuver - Moving, drawing a weapon, dropping prone, standing up, aiming, taking a guarding stance etc.
(Same as a standard and move action).
Engaged - Adjacent to object/person/thing. A requirement for use of melee weapons, martial arts and brawling. In order to interact with most objects you need to be engaged with it.
Short Range - Several meters apart. Handguns, shotguns and thrown weapons work best at this distance. Two people at short range can talk normally without raising their voice. Moving to another spot within short range is usually easy to do and generally only requires one maneuver.
Medium Range - Up to several dozen meters apart. Sub machine guns work best at this range. Two people at medium range need to talk loudly in order to hear eachother. Moving from short to medium range takes little exertion, and generally requires one maneuver.
Long Range - Further than a few dozen meters apart. Rifles, mounted weapons and gunnery type weapons work best at this range. Two people in long range needs to yell loudly in order to hear eachother. Moving from medium to long range requires two maneuvers.
Extreme Range - The farthest range at which two targets can interact. Sniper rifles and other specialized weapons may reach out to this range. Two people at extreme range may not hear each other even if they shout. Moving between long and extreme range can be time consuming and exerting, and requires two maneuvers.
Weapon types benefit or sometimes require certain distances to operate as intended. Melee weapons all need to be used in Engaged range, unless used as an improvised thrown weapon. All fire arms can be used as improvised melee weapons, but tend to be prone to breaking, extra equipment such as bayonettes can overcome this on certain weapons.
Types of Damage
Melee (Slashing, Piercing, Bludgeoning, it don't matter. A sword's blade can slash you, its tip can pierce you and its pommel bludgeon you)
Projectile (Bullets, Arrows etc)
Explosives (Grenades, Rockets and Missiles etc)
Beam (Lasers! peow! peow!)
Plasma (Energy based, hot stuff)
It's worth noting that some melee weapons use ammo, chainsaws, tasers, laser swords etc. And some rare firearms, do not! Anything explosive based though always uses ammo, usually just the 1 in the case of grenades.
DR - Damage Reduction
Clothes 0 DR