I created this quick explanation for my guild and posted it on our forum and I decided that I might as well make it a blog post since I'm sure at least one or two of you people out in the interwebs may benefit from this post. So enjoy this quick explanation of what Advanced Classes in SWTOR are and what you should know before picking yours.
First off, there are 4 basic classes: Jedi Knight/Sith Warrior, Jedi Consular/Sith Inquisitor, Smuggler/Imperial agent, Trooper/Bounty Hunter. Three of the four can tank(Warrior/Inquisitor/BH) and three of the four can heal(Inquisitor/BH/Agent), provided you select the right advanced class. Two of the four can use lightsabers(Knight/Warrior and Consular/Inquisitor) and the other two use blasters rifles/pistols.(Smuggler/Agent and Trooper/BH)
You start the game as one of these four basic classes and at level 10 you decide which advanced class you want to be. Now picking your advanced classes is not the same as picking your spec tree. What you are picking is whether you want to specialize in and once you do that, you can pick your talent spec. It sound confusing but it breaks down like this: lets say you roll a Sith Warrior and you want to be a tank for endgame.
Well when you finish your class quest on the starter world(usually around 9-10) you take a shuttle to the imperial fleet and get a quest to talk to a guy about advanced classes. The info the guy gives you about the two AC choices you have is kind of vague and not that helpful. But you are given two choices Sith Juggernaut or Sith Marauder.
The Sith Juggernaut can do one of two things: DPS or Tank. The Sith Marauder can do one thing: DPS. The choice for Marauders is how you dps with dots or with fast acrobatic dps.(think feral cat or a rogue) Once you pick your advanced class you put your talent point into the tree that corresponds to what you want to do; for our example of a tanking Warrior, you'd put your points into the Immortal talent tree. Now once you pick your advanced class and are feeling all bad ass about it, you should check your inventory because your trainer will give you a bag with items related to your AC. Juggernauts attack with a 1h lightsaber so you get two generators for your offhand, you can only use one at a time in your offhand but you get two because one is for dps and one is for tanking.
The tank one will have Shield %/Absorbtion % on it and the DPS one will only have the necessary dps stats. Kind of obvious which you should be using for your desired spec. I mention this because most people dont realize that they now have this bag in their inventory so they miss out on things like the generators for the Jugger and a second lightsaber for the Marauder to Dual Wield.
Now a very important thing to remember is that ONCE YOU PICK YOUR ADVANCED CLASS YOU CAN'T CHANGE IT! You can choose to respec your talents in your Advanced Class but you can't change the Advanced Class itself. So if you decide that you don't want to be a tank, you can respec your points and go into the dps tree. You can't just go to a trainer and choose to be a Marauder instead. You are stuck with the Advanced Class you chose, so choose wisely!
There is a good breakdown over at Reddit of the AC/Specs that also tells you what WoW class each will feel like. It's a good summary so you should totally check it out if you are not sure what AC you want to go with.
Don't Blame Me
7 hours ago