Adept: The Adept is the ultimate biotic, able to affect the physical world with the power of the mind. Adepts are durable and powerful manipulators of mass effect fields; they can use biotics to violently manipulate objects in the environment, including nearby enemy targets. Gameplay focus is on disabling and debuffing enemies while dealing massive amounts of damage.
Adepts possesses all biotic abilities, but can only equip light armor, and may only gain specialist training with pistols. They also possess no tech skills. Biotic powers focus on manipulation of dark energy with Singularity, Lift and Throw. They can weaken foes with Warp and protect themselves with Barrier.
Bastion and Nemesis are the Adept Class upgrades.
Soldier: The Soldier is a tough warrior, able to deal with a range of combat situations. The Soldier gets improved health, has the widest selection of weapons, and is eventually able to wear heavy armor. Gameplay focus is on getting into the thick of the fight, picking the right weapon for tactical situations, and outlasting opponents.
Soldiers are the full spectrum warrior of the squad whose talents focus on improving durability and damage. They may specialize in any weapon they choose and are the only class to have access to the assault rifle talent. Soldiers begin with medium armor and are also the only class that may upgrade to heavy armor. Their major weaknesses are their complete lack of biotic and tech abilities, relying on squadmates to pick up the slack in those areas.
Commando and Shock Trooper are the Soldier Class upgrades.
Engineer: The Engineer is a tech specialist, able to quickly and easily manipulate the environment with specific talents, and repair or modify technical equipment. Gameplay focus is on shaping the battlefield during combat, healing the party, and debuffing enemies (disabling weapons and lowering shields). Even though they begin the game with only pistols and an omni-tool, Engineers can unlock the ability to hack enemy technology.
Engineers possess all tech abilities, but can only equip light armor and may gain specialist training with pistols as well. They possess no biotic skills. Tech skills focus on crippling and damaging the enemy; tech proximity mine damage is, admittedly, often viewed as a secondary effect, but should not be discounted. Engineers also possess the First Aid and Medicine talents, making their healing very effective.
Mechanic and Demolisher are the Engineer Class upgrades.
Vanguard: The Vanguard is a powerful combatant, able to combine the offensive powers of the Adept and the Soldier. They have access to various weapons and armor, as well as biotics. Gameplay focus is on taking down enemies with quick and brutal force.
Vanguards are the front line of the squad. Their biotic powers focus on combat potential - disabling the enemy, making their armor ineffective, and shielding themselves from harm so that they can get close enough to go to work. The Vanguard wears light armor to start, though they can upgrade to medium armor, and they may gain specialist training in shotguns and pistols.
Destroyer and Champion are the Vanguard Class upgrades.
Sentinel: The Sentinel is able to combine tech and biotics to manipulate the environment, disable and track enemies, or defend the party. Gameplay focus is on protecting the party using kinetic barriers and healing it with advanced medical training.
Sentinels are support-focused characters, possessing biotic and tech abilities mostly focused around controlling the battlefield and keeping the squad alive. They receive no specialized weapons or armor training, but are proficient with a pistol. The Sentinel is surprisingly durable, with access to many of the shield-increasing talents, along with the Barrier biotic ability.
Guardian and Raider are the Sentinel Class upgrades.
Infiltrator: The Infiltrator is a tech-savvy warrior, able to win battles by quickly disabling and killing enemies. These soldiers focus on unlocking alternate routes, gaining access to good equipment, and obtaining an advantageous position over enemies in combat.
Infiltrators possess a mixture of combat and tech talents. They may gain specialist training with pistols and sniper rifles, begin with light armor (though they can eventually gain the ability to wear medium armor), and possess a variety of tech abilities that focus on disabling and crippling the enemy.
The Assassin and Agent are the Infiltrator Class upgrades.