Each class has a primary stat that affects their core abilities. For example, the Jedi Consular's primary stat is Willpower. Each class has a different primary stat, but Republic/Imperial classes share the same stats (Smuggler/Imperial Agent both have cunning as their primary stat for example).
Increase the damage dealt in melee combat, and increases your critical hit chance with melee attacks.
Increases your total health and the rate at which you regenerate health
Increases ranged damage and the critical hit chance with ranged weapons.
Increases the overall power of technological abilities. It affects both offensive and defensive abilities.
Increases the damage and healing strength of Force abilities, and increases the critical hit rate of force attacks.
Increases companion health, damage, and healing. This statistic only affects your companion.
Expertise increases the amount of damage and healing you do as well as reduces the amount of damage you take in PvP combat. This is a stat that appears on every PvP set.
Nearly all of your leveling gear will consist of nothing but your class’s primary stat plus endurance. However, as you reach a higher level and get access to high-end gear, more and more stats will start making an appearance.
Increases the strength of your shield, allowing it to absorb more damage.
Increases your chance to hit with attacks. Once you reach the hit cap, accuracy starts reducing enemy defenses (ability to parry/dodge your incoming attack) or resistance to force or tech abilities.
Alacrity rating is a haste effect and decreases your cast time, as well as reduces the length of global cooldown.
Armor reduces the amount of damage you take from kinetic (i.e. melee hits, many Jedi force abilities) and energy attacks (i.e. blaster fire, force lightning attacks). Does not reduce the effect of attacks which deal Internal or Elemental damage.
Increases your likelihood to get critical hits with every type of ability (i.e. melee, force, heals, etc.)
Increases parry and deflection rates. Deflect is simply parry for ranged attacks.
Increases the strength of force abilities (both offensive and defensive).
Increases the strength of all abilities, whether they use the force or not, and melee damage.
Increases the chance for your shield to become active during an attack.
Increases your critical hit bonus – i.e. crits do more bonus damage than normal hits. See also Surge Rating.
Increases the strength of tech abilities (both offensive and healing).