There are five primary/basic character attributes in SWTOR: Endurance, Aim, Cunning, Strength and Willpower. Endurance gives you 10 HP per single point, and the other four stats increase your class’ base damage and critical chance:
All primary stats give 1% crit for every 140 (at level 50)
Aim gives 0.2 ranged damage
Strength gives 0.2 melee damage
Cunning gives 0.2 tech damage
Will power gives 0.2 force damage
Secondary Statistics:
Presence – boost companion health, damage and healing
Power – adds melee, force and tech damage
Force/Tech Power – found on weapons, works like Power but does not provide melee/ranged damage
Accuracy – Gives +hit and gives armor/spell pen over 100%
Crit – gives melee, force and tech crit
Surge – increased critical damage
Armor – reduces physical and kinetic damage (all Tech and Force powers unless listed otherwise are Kinetic)
Defense – increases Parry/Deflect (deflect is just ranged parry)
Shield – increases chance to be shielded on attack
Absorption – increases the % of damage shaved off shielded attacks
Alacrity – cast/channel Haste
Expertise – PvP stat. Increases damage and healing done, reduces damage taken.
A lot of people (in various forums) seem to be debating which healer is going to be best, so to break it down a bit based off their skills and skill trees I feel its safe to say:
Once you choose an Advance Class (AC), it cant be undone, but you CAN respec your skill trees for any one AC. keep this in mind when choosing. All of the healer classes have different resource systems, but each of them has at least one small, spammable, "free" heal that doesn't use any resources. It won't be enough to get through tight spots, but it will make resource management tolerable.
SI sorcs will have castable damage shields (think wow priest PW shield) and channeled or cast timed heals (mostly reactionary) with a recharging, mana-esque resource. It has a battle res. Its dps respec is a typical caster: cast timed burst spells, as well as some channeled DoTs and large aoes.
IA opers will have a single target HoT heal and then cast timed and reactionary instant heals with a constantly recharging energy system (think wow rogues). It has the ability to stealth and it has a battle res as well. Its dps respec is a melee class that can specialize in burst damage or poisons. It has some ranged skills, but not enough to keep it out of melee range effectively.
BH mercs will have a reactionary healing shield castable with multiple charges on a single target (think shammy earth shield from wow). and then the typical healing skill set. It uses a heat system that builds up with skills and dissipates over time, with a few mechanics to help lower it back down and a number of heat-less skills to keep it low. It does NOT have a battle res. Its dps spec is a ranged class that has some CCs and can either burn through all its heat in a quick burst or use a more sustained rotation to keep heat low (pvp vs pve maybe?)
It appears you can pick and choose from the specialization trees, but i doubt you will see such a mix too much in pve, but will likely be prevalent in pvp for a mix of survivability and dps.
I know some people only care about numbers, but for those who care about how a class plays, I hope this helped.