Shield boosters convert energy from your capacitor into shield power, meaning it gives you more shield while taking from your capacitor power.
Shield extenders boost the strength of the shield.
Shield flux coils increase the shield recharge rate but lowers your maximum shield capacity.
Shield hardeners, are in my opinion, the best to use. They give increase to damage resistances.
Shield power relays increase the shield recharge rate by diverting power from the ship's core, meaning it takes from your capacitor power.
Shield rechargers give an increase to the shield recharge rate. And Shield resistance amplifiers strengthens damage resistances.

Armor tanking is a bit different. Instead of focusing so much on shield strength and resistances, you look at armor strength and resistances. While shield tankers have shield boosters and boost amplifiers, armor tankers have armor repairers, but one thing both shield and armor tankers have in common is the need for CPU!

While shield tankers focus on Engineering, armor tankers are all about Mechanics. This is the skill category to take a look at when armor tanking. The most important skills to have for armor tanking are: EM Armor Compensation, Explosive Armor Compensation, Kinetic Armor Compensation, Thermic Armor Compensation, Hull Upgrades, and Repair Systems.

The first four are pretty self explanatory. They give damage resistance to each type of damage, just like in shield tanking. Hull Upgrades allows you to maintain your ship's armor and install hull upgrades such as cargo expanders and inertial stabilizers. It also gives a bonus to your armor strength. Repair Systems is very important because it allows you to operate armor or hull repair modules. It also gives a reduction in repair systems duration.

Some other skills to have for armor tanking are: Energy Grid Upgrades, Energy Management, and Energy Systems Operation.
Energy Grid Upgrades allows you to install power upgrades such as capacitor batteries and power diagnostic units. It also gives a reduction in CPU needs of modules requiring the skill.

Energy Management and Energy Systems Operation are also used in shield tanking. Energy Management gives a bonus to your capacitor's capacity and Energy Systems Operation allows you to use modules such as capacitor boosters.

Once you have the right skills, you need the right modules. This is where armor tanking differs greatly from shield tanking. Before we talked about different powered slots on your ship. Shield tankers concentrate on medium power slots. Well, armor tankers concentrate on low power slots. Here is a list of the most important modules in armor tanking. Armor hardeners, Energized plating, and resistance plating all give a bonus to damage resistance. Armor plates give an increase to your armor strength. Armor repair systems, are in my opinion the most important module in armor tanking. It allows you to repair your armor. Damage Controls give a bonus to resistances for shield, armor and hull.