At a glance, the stats in Star Wars: The Old Republic are simple. There are 4-7 primary stats (depending on how you view it) and they are fairly easy to understand. You view your character in the game and see those stats, and can mouse over them to see what effects they have. Simple enough.
Except that, for one, there are 4 to 7 stats, depending on how you view it. And then there are ... many more that are not listed at all in the character profile. This post is to talk about all of those, and why the primary stats are weird.
As mentioned, there are between 4 and 7 primary stats, depending on your take on what Primary means. I'll be splitting these up here to explain why.
These stats are increase the effectiveness of the character's abilities, and are specific to individual classes. They increase damage, crit and healing. This works for a base class, and any advanced classes attached to it. For instance, when you play a Jedi Consular and then become a Jedi Shadow, your damage is still going to be based on your Willpower.
Aim is for Troopers and Bounty Hunters
Cunning is for Smugglers and Imperial Agents
Strength is for Jedi Knights and Sith Warriors
Willpower is for Jedi Consulars and Sith Inquisitors.
Other Primary Stats
These stats are not specific to any individual class, but instead are effective for all of them. Though not necessarily in all circumstance.
Endurance is the fifth primary stat. It increases your health, and thus the amount of time you get to fight before dying. It stands apart because it is valuable to all classes, making it perhaps the most primary of stats.
Presence does not actually affect your character. It instead increases the effectiveness of your companions. It will increase their health, damage and healing, in addition to the stats on their equipment. As such, you may find that your companions are more capable than you, numerically. Presence, however, does you no good for PvP or any circumstance in which you are not fighting with your companion.
Expertise increases your damage and healing, as well as reducing the damage you take, but only in PvP. It is of no use when you are not fighting other players.
These are the statistics that don't appear in your character profile, but can have a profound effect on your ability to decimate/survive your enemies. These will begin to appear on blue quality or greater items, and also as Augments which you can apply to Exceptional quality crafted items. They can be separated into a few types.
Absorption is a damage mitigation stat. When you are hit by an attack and have a shield, as tank classes do, your shield has a chance of becoming active and absorbing/mitigating some of the incoming damage. The Absorption stat increases the amount of damage your shield will absorb.
Defense increases your chance to parry or deflect an incoming attack.
Shield rating increases the chance for your shield to activate when you are hit..
Accuracy increases your chance of hitting an opponent. When your chance to hit reaches 100%, further Accuracy will decrease their chance of parrying or deflecting.
Alacrity decreases the amount of time your abilities take to cast. It also decreases the global cooldown, which is the 1.5 seconds you must wait between activating one ability and the next.
Critical rating increases your chance of getting a critical hit when activating an ability. This affects both attacks and heals.
Power comes in a few varieties, but in the end they are all the same, so I will summarize in this entry. Power increases the effectiveness of the appropriate abilities. For the core Power ability which is more rare, it will improve all abilities, whether Force or Tech based. Force Power and Tech Power will appear on items as well, and work exactly the same, but they only improve abilities appropriate to them, with Force Power affecting Force-based abilities and Tech Power improving Tech-based abilities.
Surge rating increases the effect of your critical hits. The standard effect for a crit is that the attack/heal does an additional 50% damage/healing. Surge increases from that +50%.
There You Have It
The Secondary stats are not particularly intuitive, and are not going to show up when you are first leveling a character. Almost all green drops will only have the appropriate primary stats for a class, plus endurance. As such, you'll be confused when the Secondaries start showing up. But when you are actually trying to tank flashpoints or operations at later levels, you'll need to know your defensive stats and build your gear appropriately. I hope this proves helpful to you, as it was nice to figure out for my own edification.