(covers information from several alternate timelines)
Defensive systems were any device or technology that was used for protecting the ship and crew. These included point-defense systems, shields, force fields, weapons, and to a lesser extent cloaking devices, hull plating, and in some cases uniforms.
Starships are the capital vessels of civilizations, and thanks to their extended missions in deep space far from outposts and engineering facilities, mount weapon systems which are explicitly designed to provide defensive forces to protect the starship from damage. There is a certain amount of overlap in the activity of defensive systems and offensive systems, though the core mission of any exploratory starship is one of peace, and thus the use of defensive system for offensive purposes will be counter the main mission of the vessel.
Proximity sensors mounted on the hull were slaved to ship's computers, and were able to detect approaching objects as well as energy impulses, evaluate these threats to shipboard integrity, take action (in situations where Human judgement time was insufficient, as defined by software interrupt programming) in the form of maneuvering thruster fire, phaser burst, automated photon torpedo fire, or in extreme cases, a temporary jump to warp. The last of these was the most unlikely, but potentially possible, and would have been of extremely short distances.
In all cases of point defense system action, crew members would be informed of events either in the form of alerts, or in the form of damage reports (where damage occurred before systems had time to report), or else of ship system actions in response to threats, with command option overrides to provide the crew time to adjust to circumstances.
Shields served to protect the hull of a starship from transmission of hard radiation, space dust, and provided a quantum separation of the ship from hyperspace when performing a warp jump. Shields were able to repel the impact of photon torpedoes to a certain extent, though they were unable to fully compensate for a barrage of weaponry aimed at single points on the hull. Shields were also unable to compensate for significant masses making contact with the hull of a vessel, as their function was not one of imparting an inertia to objects.
Shields functioned extremely well in the line of defense in deflecting energy pulses by phasers in breaching the ship's hull.
Force fields were a relative of an energy shield - where a shield was a hull-conformal energy field able to deflect high energy beams, a force field was a man-carryable device, also mountable on field installations and small vessels such as shuttle craft. Typically, the force field was functional only in deflecting energy pulses, by refraction rather than by reflection. It was energically expensive to operate, and vunerable to damage in battlefield situations.
Shipboard weapons were divided into two classes: phasers and photon torpedoes.
Phasers are high energy photon weapons whose discharges are highly destructive to matter they encounter. Cannons are mounted on positions on the hull of starships from which phaser beams are fired under direct computer control. Phaser beams are visible in space thanks to their density of ions accompanying a firing burst, and from their localised disruption at surface contact. Phasers are primarily a hull point-defense weapon, and are equally the offensive weapon of choice thanks to their high destruction/energy cost ratio.
Photon torpedoes are self-homing unguided targeted weapons. These are primarily defensive weapons designed to permit a starship to protect itself from large masses approaching at velocity in normal space, such as asteroid and cometary fragments. Terminal guidance is typically by maser, which is what gives the explosive warheads their name. Many different kinds of explosive cartridge can be mounted, and the most powerful of these are matter-antimatter charges.
Phasers dominate in terms of personnel weapons. The typical phase-pistol has a mechanised emitter, with two lenses. A large aperture green lens, when facing forward, distributes a stun pulse of phaser fire. By moving a thumb-operable a switch on the body of the weapon, the emitter will flip to enable a smaller red lens, which supplied energy suitable to kill a sentient being, or vapourise objects of masses up to 350kg at specific densities. Energy for the quite compact phaser elements is stored within the body of the weapon.
For most shipboard action, hand held unit suffice, though it is expected that assault phaser rifles exist for use by larger boarding parties.
Favored by the Vulcan, Romulan, and Klingon races, cloaking is a defensive adaptive camouflage for vessels. Cloaks typically employ holographic projects, or else a refractivity of the conformal shield that conducts all photons around the vessel as though it were not there. Cloaks are typically highly effective, as they do not betray the presence of a vessel to photon-based sensor systems. However, all good stealth systems possess flaws, and cloaks are no different. Plasma exhaust from thruster and impulse drive systems are amenable to detection by suitable sensors. For cloaks embodying refractivity, exotic particles like tachyons, and penetrative particles such as electrons (which do not share the same refractive index as photons) can be used to pierce a shield system.
Analogous to the techniques of fortifying land transport weapons systems such as battle tanks, hull plating serves a very sincere defensive purpose. Space is replete with micro-meteors, which over time will abrade a ship's hull. Energy shields conformal with the hull are vital, as is the deflector dish emitter at the prow of a ship, in preventing these particles from damaging the hull at near-relativistic speeds, and in the transient to the warp jump, where ship speed is very great for an instantaneous time. Though 90% of the particles a ship encounters will be dealt with this way, fully 10% will not. Hull plating serves to defend the ship from these small interloping particles. It is replaced on a regular basis when ships are docked at a starbase.
Environment suits are particular examples of defensive personal uniforms. The best examples are the suits designed to be used for space jumps, which are flexible and yet tough enough to protect occupants from the transition from space vacuum to atmospheric pressure. Uniforms are defensive in being able to accommodate defensive weaponry for the ship's officer.
Physical barriers were a type of the defensive system of a prison of the Mokra Order on a planet in the Delta Quadrant. Kathryn Janeway asked for physical barriers, detection grids and force fields to gather information about the prison from Caylem. (VOY: "Resistance")