Not all ships in all fleet settings require every single kind of defensive modules, if any at all. Depending on the size and composition of your fleet, you can install a few modules to drastically increase your survivability in the event of PVP ambush by facilitating evasion or escape. Some ships (namely those using small T1 hulls) are so cheap they are considered expendable, and it is not worth degrading their performance by swapping out useful modules for defensive ones.
Always obey Rule #1 of EVE: Never fly what you cannot afford to lose! This also includes the value of your ship’s holds
Using Warp Core Stabilizers
Every ship has an attribute called “Warp Core Strength”. For most unmodified ships, the starting Warp Core Strength is 0, though some ships come with a Warp Core Strength bonus (eg. +2 for the Venture). In order to use your warp drive, you require a Warp Core Strength. When ambushes, enemies almost always use Warp Disruptors or Warp Scramblers to lower your Warp Core Strength in hopes of dropping it below 0, thereby preventing you from escaping via warp. Warp Disruptors generally have a range of 20km, and deduct 1 point from your Warp Core Strength. Warp Scramblers generally have a range of 7.5km, deduct 2 points from your Warp Core Strength, and also prevents the use of your Microwarp Drive and Microjump Drive((I do not know if you can still use your MWD or MJD if you manage to keep your warp core strength at or above 0 when you’re scrammed.)).
One way to overcome the warp penalties is to install Warp Core Stabilizers, low slot modules that provide 1 point of Warp Core Strength each. It is recommended that a minimum of two be used as that would also ward off the effects of a Warp Scrambler whereas a single Warp Core Stabilizer would not.
Note that Warp Core Stabilizers do carry heavy penalties to maximum targeting range and scan resolution; however, for many ships, these penalties have no relevance to the ship’s purpose (eg. transport ships), have minimal impact on the ship’s purpose (eg. mining vessels), or can easily be overcome with sensor boosters/amplifiers (eg. solo PVE long-range ships).
Using Cloaking Devices
Cloaking devices are high slot modules that, once activated, prevent the enemy from seeing or targeting you; therefore, they cannot directly harm you or prevent you from warping. Once cloaked, the only way to decloak is if you manually deactivate the cloaking device, or if you find yourself within 2000m of a non-cloaked object. Thus, if an enemy knows your general location, they might fly around in hopes of ‘bumping’ into you, thereby forcing you to decloak.
There are two types of cloaking devices: Covert Cloaking devices and Non-Covert Cloaking devices. Covert Cloaking devices permit you to warp while cloaked; non-covert cloaking devices cannot be used while warping or attempting to initiate warp. Only a select few ships are able to use Cover Cloaking devices; they will be explicitly mentioned in their info window. If a ship’s info window does not explicitly mention a Covert Cloak device, then a Covert Cloaking device cannot be used though a Non-Covert cloaking device can.
On most ships, activating a cloak incurs heavy penalties, primarily to max velocity (90% reduction in the worst case). It suffices to say that, for the purposes of surviving an ambush, cloaking and moving away from your known position may cause the enemy to give up trying to find you; however, they may also camp the site waiting for you to uncloak, or they may try to fly around your last known position in hopes of bumping you. If you activate a Microwarp Drive within the first 5 seconds of initiating cloak, you can overcome much of the max velocity penalty, and put more distance between you and your last known position. It is advised that you move directly up or directly down from your last known position. Lead the enemy astray but moving in one direction while partially visible, and then move to a completely different direction once fully cloaked.
Note that being invisible does not make you invincible; you may still suffer the effects of ECM, Smartbombs, interdiction spheres, etc; however, none of these Area-of-Effect modules will cause you to decloak. For example: when using a covert cloak, an interdiction sphere might stop your warp, but then you could simply fly through an enemy gatecamp towards the gate; as long as you don’t bump into anything, you will not be decloaked.
Also note that you cannot cloak if you are being targeted or in the process of being targeted. If an enemy appears on your overview, you should cloak before they get the opportunity to prevent you from cloaking.
Using HP Buffers
The purpose of using HP buffers (eg. shield boosters, armor plates, hull bulkheads) is to give you enough health points to survive long enough to survive an ambush. Having a high HP pool enables you to survive the enemy’s alpha strikes while your guard is fighting off the enemy and/or until you have aligned and accelerated to warp. Even if you have a guard capable of repelling an enemy, or have the ability to overcome an enemy’s warp jamming, if you cannot survive the skirmish, or survive along enough to enter warp, then there’s no point in using any other defensive module (eg. warp stabilizers).
In general, the best strategy to adding buffers to do use whatever modules will maximize your minimum Effective HP. Generally, it is best to first start with buffers, then resistance amplifiers that would boost your minimum HP more so than additional buffers. Repair units are generally not desirable unless you have a large HP pool, and you believe you will survive long enough to make a repair unit more so worthwhile than an additional buffer.
Electronic Warfare and Capacitor Warfare can be used defensively as well as offensively. ECM or Sensor Dampening EWAR modules can break or greatly diminish the enemy’s ability to destroy you or prevent your escape. Typically, EWAR is best left to dedicated EWAR ships, but in some cases it may be worth while for a non-dedicated EWAR ship to use them defensively.
Likewise, Capacitor Warfare uses energy nosferatus and neutralizers to diminish the enemy’s capacitor, thereby diminishing the enemy’s ability to destroy you or prevent your escape. In particular, the enemy will be unable to use warp jamming modules, hybrid weapons, and energy weapons without sufficient capacitor reserve. Typically, CAPWAR is best left to dedicated EWAR ships, but in some cases it may be worth while for a non-dedicated CAPWAR ship to use them defensively