There are six classes:
Adept - Biotic specialist
Soldier - Combat specialist
Engineer - Tech specialist
Vanguard - Biotic/Combat
Sentinel - Tech/Biotic
Infiltrator - Tech/Combat
Sentinel class is an all -around versatile class, however, they only get to use pistols in the first game, and short-range weapons in 2 and 3.
Vanguard class is a frontline class with shortrange weaponry.
Infiltrator class is a tech/combat class good at sniping and sneaking. They can use shortrange automatics, pistols, and sniper rifles. They have access to disruptor and cryo ammo.
Engineer class is the ultimate tech class, but is not to my playing style.
Adept class is the ultimate biotics class, but it's weakness because of it makes it something I would not normally choose as my main class.
Soldiers are the ultimate combat class. They have access to all weapon types and in 2 and 4, they have access to all ammo types. They also need to rely on their squad mates for biotics and tech abilities.
Ammo Types:
Disruptor ammo: Electrical powered bullets. Overload shields faster and more damage to synthetics. Also, there is a chance to disable weapons on enemies.
Incendiary ammo: Fire powered bullets. This does extra damage over time to organic enemies. Synthetics are resistant to fire damage. Ignited enemies have their health regeneration stopped. Fire damage weakens armors. This may also cause enemies to panic.
Cryo ammo: Ice type ammo. This ammo is exclusive to Shepard. Chance to freeze enemies, weaken armor, and decrease movement speed of afflicted enemies.
Liara T'Soni is an Adept in all three games. She has access to electronics and access to an omni-tool.
Garrus Vakarian is an Infiltrator in all three games. He has acces to electronics and the omni-tool in the first game.
By having Garrus in the party, I can almost guarentee that I will have someone with tech abilities.
The thing that I like about biotic classes are their throw abilities.
The Vanguard and Sentinel classes are closest to how I want to play.
Vanguard: Biotic/Combat
Sentinel: Tech/Biotics
When compared side-by-side, it is clear that Sentinel is the path for me if I am to do a biotics class.
The Infiltrator is also a good class to choose for more weapons choice as the sentinal is very limited on weapons.
Something interesting to note is the the Sentinel class talent provides the pistol line benefits since the Sentinel doesn't have the pistol line.