Depending on the Star Wars the Old Republic classes that you have, you will have to focus on different stats. I will go over the different stats and what they do for you in game. Lastly I will point out which stats for you to add if you use a certain SWTOR class. This guide will shed light on all the stats mentioned in game to help you get started easily.

SWTOR Primary Basic Stat

There are five primary stat in SWTOR: Endurance, Aim, Cunning, Strength and Willpower. They will do the following for you.

Aim: 0.2 ranged damage
Strength: 0.2 melee damage
Cunning: 0.2 tech damage
Willpower: 0.2 force damage
Endurance: you 10 HP per single point

Special Note
All primary stats give 1% crit for every 140 points spent at level 50.

SWTOR Class Primary Stat Bonus

Depending on the class that you have, you will have to focus on specific areas of primary stats. These focus will help you to deal additional damage with your SWTOR classes. The classes here are noted along with their advanced class counterparts.

Empire

Bounty Hunter (Mercenary/Powertech): Aim
Imperial Agent (Operative/Sniper): Cunning
Sith Inquisitor (Assassin/Sorcerer): Willpower
Sith Warrior (Juggernaut/Marauder): Strength

REPUBLIC

Trooper (Commando/Vanguard): Aim
Smuggler (Gunslinger/Scoundrel): Cunning
Jedi Consular (Sage/Shadow): Willpower
Jedi Knight (Guardian/Sentinel): Strength

Additional Misc Character Primary Stats

These are some of the stats that are not that important but nevertheless primary.

Presence: Boosts companion health, damage, and healing.
Expertise: Boosts your PvP combat effectiveness.
Social: Social points.
Valor: Exp gained through PvP and point can be used to purchase PvP gear.

Secondary Stats

These are secondary stats that you would see in game. I have divided them into these categories for easier navigation:

Offensive

Power: melee, force and tech damage
Surge: critical damage
Force/Tech Power weapons only, similar to Power but does not provide melee/ranged damage
Crit: melee, force and tech crit
Accuracy increase hit

Defensive

Armor: reduce physical and kinetic damage (all Tech and Force powers are Kinetic unless stated otherwise)
Defense: increase Parry and Deflect
Shield: increase chance to shield an attack
Absorption: increase the % of damage absorbed on shielded attacks

Other

Alacrity: Increase Cast/channel speed
Presence: companion health, damage and healing
Expertise PvP stat. Increases damage and healing done, reduces damage taken.