As already stated, there are six classes available to Shepard. Each squadmate also represents one of the classes, although the aliens have some added perks that the humans don’t. At its core, the class system is a successful futuristic upgrade of the holy WRPG trinity of Warrior-Rogue-Mage. Each class is proficient in one (‘basic’ classes) or two (‘mixed’ classes) of three domains: combat, techs or biotics. The idea is that the basic classes are more focused, with a full array of skills in their chosen domain, while mixed classes are more versatile, at the expense of some of the more advanced abilities (like D&D multiclassing, if you will). Techs are electronic skills aimed at disabling machinery. This includes cracking safes, tampering with enemy weapons, but also security turrets, drones and geth. Biotics are telekinetic abilities developed by most species from exposure to Element Zero (except asari, among whom they occur naturally) and enhanced via neural implants. Those can really wreak havoc among enemy forces. Combat happens with guns: pistols (accurate, but weak), assault rifles (medium range, but not very accurate), shotguns (powerful, but inaccurate and short range) and sniper rifles (very powerful, but slow and long range). It’s a good idea to have a mix of all three types of skills on your team, which should inform your choice of squadmates. Once Shepard hits level 20, s/he can also pick one of two specialisations for her/his chosen class, one defensive, one offensive. The exceptions are Infiltrator and Sentinel, which have two offensive and two defensive specialisations, respectively. In most cases, it’s better to go the offensive route, except perhaps for the Adept (and the Sentinel, obviously).
Soldier: The most beginner-friendly class, this is the equivalent of the Fighter/Warrior of oldschool RPGs. The Soldier focuses on taking and dealing physical damage, with the downside that it can get rather boring. It has the best HP of the lot, with the ability to regenerate, can wear any type of armour and use any weapon, including assault rifles (which, otherwise, only Garrus and Wrex can use). Each weapon type has its own associated special ability, learned as Shepard specialises in it. E.g. Sniper rifle specialisation grants Assassination, a dramatic boost in damage and accuracy, which completely cancels the targeting drift. At level 20, the Soldier can specialise as a Commando or a Shock Trooper. The Commando is the offensive specialisation, granting a boost to weapon damage and improving Assassination and Marksman (the pistol skill, which increases fire rate and accuracy, while reducing heat generation), but also the Immunity skill (reduces damage taken). The Shock Trooper is the defensive specialisation. It also improves Immunity, while reducing the recast timer of Adrenaline Burst, which resets the timers on all other abilities for faster use. Among the squadmates, Ashley is a Soldier.
Engineer: The basic class that focuses on techs, this is the equivalent of the Rogue from oldschool RPGs, as it can use Decryption and Electronics to hack locked containers and such. The latter is done via a minigame that you can fail. If that happens, you’ll need omni-gel to bypass the lock. The Engineer has an omni-tool, a yellow holographic interface that looks like a glove and which it uses to launch a variety of proximity mines with negative effects. They can overheat weapons or blow up canisters of biohazardous material (Sabotage), strip enemies’ shields (Overload), prevent them from using biotics (Damping) or even hack AIs to make them attack each other. They also have enhanced squad healing abilities and better tech protection. The downside is that they can only equip light armour and pistols. At level 20, the Engineer can specialise as a Medic or an Operative. The Medic is the defensive specialisation, improving Neural Shock (deals toxic damage to enemies) and First Aid (better healing). The Operative is the offensive specialisation, reducing the recharge timers for all tech attacks and improving the blast radius of Overload and Sabotage. This is a pretty ‘humble’ class: its abilities are useful, but not flashy, and it won’t be providing much straight-up damage. Among the squadmates, Tali is the closest thing to an Engineer, except she can also use shotguns, which makes her more apt at combat.
Adept: The basic class that focuses on biotics and my personal favourite, this is the equivalent of the Mage in oldschool RPGs. It’s also extremely effective in this game. Its primary task is to fling things across the battlefield, be it enemies or objects. Lift and Throw are self-explanatory; at maximum level, Lift will even work on geth colossi. Singularity creates a globe of energy which draws everything in the vicinity towards it, Barrier strengthens shields, while Warp lowers defences and deals damage over time. Stasis freezes an enemy, but also makes it impervious to damage, meaning that it’s only useful if you become overwhelmed. The flipside of all this telekinetic power is that the Adept can only use light armour and pistols. At level 20, the Adept can specialise as a Bastion or a Nemesis. The Bastion is the defensive specialisation: it improves Barrier and increases its duration, and removes the invulnerability from Stasis, which makes it very useful when facing large dangerous enemies. The Nemesis is the offensive specialisation, increasing the damage and duration of all biotic abilities and improving the radius of Lift and Warp. Bastion is probably the only defensive specialisation really worth considering over its offensive counterpart, especially on higher difficulties. Among the squadmates, Liara is almost identical to an Adept, except that she can also use Overload.
Infiltrator: The mixed class that combines the skills of the Soldier and the Engineer, with both combat and tech proficiency. It can use medium armour, pistols and sniper rifles, which makes it proficient at long-range combat, i.e. the opposite of the Vanguard. Its tech skills include Sabotage, Overload and Damping, and it can hack locks and containers. The Infiltrator is ideal in situations where you can sneak up on enemies, as it can effectively clean out large stationary threats like geth colossi, rocket turrets or lookout troops. However, it will be at a disadvantage when enemies close in. Normally, I’d be all over this class, as I have a fondness for sniping, but, for some reason, it feels less streamlined in this game than in ME2. Perhaps because you need to put a lot of points into sniper rifles before the targeting drift stops being handicapping. Still, there are some situations where picking stuff off at a distance helps immensely (looking at you, Antibaar geth outpost). At level 20, the Infiltrator can specialise as a Commando or an Operative, which work just like their Soldier and Engineer counterparts. The main difference with the other classes is that these are both offensive specialisations. The choice is mainly between more weapon damage and more accuracy (Commando) or more tech damage (Operative). I’d say the tech damage is more beneficial at higher difficulties, when enemies don’t go down easily, so disabling them is more advantageous. Among the squadmates, Garrus is the closest equivalent to an Infiltrator, with the difference that he can use assault rifles, and that his Overload isn’t immediately available.
Vanguard: This is a mixed class that combines the skills of the Soldier and the Adept, giving it both combat and biotic abilities. It can use medium armour, pistols and shotguns, which makes it proficient at close-range fighting. Its biotic arsenal consists of Throw, Lift, Warp and Barrier, so the only useful ability it’s missing is Singularity. The Vanguard will be at a disadvantage at long range, especially when dealing with turrets or enemy snipers. However, this is necessary to balance the class out a bit, because otherwise, you’d just be in god mode for the entire game. At level 20, the Vanguard can specialise as a Shock Trooper or a Nemesis, which function just as their Soldier and Adept counterparts do. Shock Trooper is the defensive specialisation, which improves Immunity and Barrier, and reduces the recast timer of Adrenaline Burst. Nemesis is the offensive specialisation, which increases the damage and duration of all biotic abilities, as well as the radius of Lift and Warp. Among the squadmates, Wrex is the closest you get to a Vanguard, except he can use assault rifles instead of pistols (thus having a longer range of attack) and wear heavy armour for better damage protection. He also has his krogan regenerative abilities as an added perk. To compensate for all that, he swaps the very useful Lift for the far less useful Stasis.
Sentinel: The final mixed class combines–you guessed it–the skills of the Engineer and the Adept, with both techs and biotics at its command. This is the ultimate support class, as it can only use light armour and pistols, but has Throw, Lift, Barrier and Stasis on the biotics side, and Overload, Sabotage, First Aid and Neural Shock on the tech side at its command, with the ability to hack locks and containers as well. So, all in all, more defensive skills than either of the classes it combines. Still, due to its selection of biotics, you can play it more offensively than an Engineer and so make things more exciting for yourself. At level 20, the Sentinel can specialise as a Bastion or a Medic, which work just like they do for an Adept or an Engineer, with the peculiarity that they’re both defensive specialisations, just to further drive home the idea that this is supposed to be a support class. The choice is between improving Shepard’s biotic defences (Bastion) with the same Stasis perk as the Adept, or her/his tech defences (Medic). Among the squadmates, Kaidan is a Sentinel.