I'm not going to get into a philosophical discussion about tanking types (shield vs. armor and passive vs. active), but rather address this from a factual standpoint, so you can tank your ship however you please, hopefully better with the knowledge I provide you.
In EVE, HP and EHP are almost never identical. Total hit points (HP) are the combined raw shield, armor and structure points your ship has after any physical modules or rigs are added to provide additional points. HP is almost never equal to the damage you can take (or tank) though. There is basically one module and one rig (per defensive type) that increases your HP. Shield Extenders, Armor Plates and Reinforced Bulkheads are the modules, while Core Defense Field Extenders and Trimark Armor Pumps are the rigs. You will notice that there are no Hull rigs. That's because nobody really hull tanks. You just shouldn't. Installing these modules and/or rigs to your ship increases the base HP of your ship.
Effective Hit Points (EHP) is the combination of HP and resistances. I am going to talk about EHP as a static number, even though it's not. EHP varies from engagement to engagement, and , so please take the time to think about your EHP every time you engage, whether in PvP or PvE.
We are going to use a Gallente Thorax as an example. Below are the raw statistics of the Thorax:
|Hit Points||Resists (EM/Exp/Kin/Therm)|
This ship (before any modules) has 5040 HP, but 6225 EHP vs. kinetic damage. That means if a single volley of damage were 5500 DPS (kinetic only) the ship would survive with a fraction of hit points (725 structure points).
How EHP works (in a perfect world).
Your ship can take a certain amount of damage (assuming no repair services) before it blows up. The more damage it can take (or mitigate) the longer it lasts before it explodes. Each ship has some basic resists to each of the four damage types, and some "holes" to some of the damage types.
In a simple world an 80% kinetic shield resist Thorax can withstand 6833 DPS of kinetic damage. In the real world of EVE, no weapon puts out a single damage type. Hybrids do Kinetic/Thermal damage, Lasers do EM/Therm, while missiles and projectiles do varied damage types. So most ships fit for at least two types of resistance.
Your EHP should be optimized to defend against the types of ships you will engage. For PvE this is actually pretty simple. PvE ships should fit against the (typically 2-3) damage types they will face. If you run missions (or rat) against Serpentis, that means you want to have strong resists against Kinetic and Thermal damage, since Serpentis rats usually utilize hybrid weapons. In PvP it gets more complex. A lot of PvP players fly Minmatar to provide them the opportunity to optimize damage against any opponent they encounter. Traditionally PvP ships try to balance resists across the board, recognizing that (for example) plugging the EM shield hole in Gallente is critical, because a good PvP pilot will utilize EM-focused damage against your shields, in anticipation of it being the weakest resist. However, if you know you are going to battle a fleet of Oracles, it's a safe bet to maximize your resists against EM and Thermal damage.
I don't want to get into tanking types, but just a brief comment so you understand that any repair modules affect your EHP. If you are active tanking, it's important to understand that your repairs are worth more than the raw numbers, due to EHP as well. For example, if you fit a Medium Ancillary Shield Booster on that Thorax, it repairs 146 shield hp every 3 seconds, but that 146 HP is actually worth 204 EHP (kinetic) in a basic Thorax in combat.
I hope this brief overview into how EHP works helps you fit your ships for better survivability.