So ... this is going to be split into sections and sub-sections.
I'll keep each one of them brief, as nobody likes reading for 20 fucking minutes, with just enough details to get you going.
USCM:
The United States Colonial Marine force.
Roles:
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Private / PFC:
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You're a rank-and file marine
Stick together with your SL or other marines.
Contribute to the battle by being the front-line.
FOLLOW ORDERS FROM ALMOST ANYONE.
Engineer:
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Create barricades and fortified positions using metal, plasteel, and sandbags with barbed wire.
You can also use turrets, tesla coils, and other such unique devices.
Make safe places for medics and doctors to treat patients.
Medic:
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Drag wounded marines away.
Treat wounded / dead / dying marines.
Smart-gunner / SG:
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Capitalize off of your IFF features by allowing PFC's / privates to push in front of you while you provide heavy fire support from the rear.
DO NOT get killed!
Specialist / Spec:
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A special unit that can use a special weapon / equipment loadout to the marine side's advantage.
Scout = Cloakable with long-range rifle
Sniper = super-long-range rifle with more damage
Grenadier = auto-grenade launcher
Demolitionist = RPG / SADAR specialist
Pyro = Use the power of the blue flames and your immunity to flames to turn a xeno push into a screeching halt, as well as deal HEAVY damage over-time to xenos ignited by your flames. Beware however, that queens and ravagers are IMMUNE to fire.
DO NOT GET KILLED!
Squad Leader / SL:
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Lead the squad, listen to command's orders as they have a higher vision for the battlefield plan, and relay it to your squad and get them to follow you.
Call in CAS Dropship strikes with direct drop or directional strafe with a red laser designation.
Call in Orbital Bombardments using co-ordinates found by using the green laser on your laser designator.
DO NOT GET KILLED!
Doctor:
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Do surgery on marines that have broken bones, internal bleeding, or that have organ damage.
Use the wiki to determine what tools are needed and when to use them for surgery.
The Chief Medical Officer, or CMO, is your superior and authorizer.
Researcher:
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Create and invent new medicines and drugs to help the marines and doctors ground-side from super-healing chemicals, to chemicals that can auto-revive and heal broken bones.
Use Queen's blood / praetorian blood with a captured, un-planted egg to grow your very own corrupted, marine-friendly hive to fight the xenos!
The Chief Medical Officer, or CMO, is your superior and authorizer.
Requisitions / Cargo Technician:
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Use the Almayer's Cargo hold and your gradually increasing credits to purchase supplies for the marines.
Whether it be metal, plasteel, ammo, specialty ammo, or other-wise.
The Requisitions Officer, or RO, is your superior and authorizer.
Maintenance Technician / MT:
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Repair and maintain the ship's systems, engines, and structural integrity.
The Chief Engineer, or CE, is your superior and authorizer.
Military Police / MP:
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Maintain and enforce marine law and order within the USS Almayer.
Make arrests.
The Chief of Military Police, or CMP, is your superior and authorizer.
Pilot Officer / PO:
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Take care of and maintain the dropships.
Make transport runs to and from the surface of the Area of Operation (AO).
Advanced knowledge can make you a Close Air Support (CAS) PO, dropping ordinance where marines need it.
The Bridge Crew (SO's, XO, CO) are your superiors and authorizers.
Weapons (basic):
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m41a pulse rifle:
decent damage, decent fire rate.
Has normal, extended, and AP magazine variants.
Shotgun:
medium to high damage, low fire-rate.
has Flechette (med-range AP and armor shred), Slug (Long-range stun/silence), and Buckshot (close-range high dmg + stun) for ammunition variants.
M39 SMG:
low-medium damage, VERY high fire-rate.
Has normal, extended, and AP magazine variants.
Pistols / Sidearms:
Mod88: Decent, but weak against un-armored targets.
M4A3 Pistol: Weak, unless AP or Hollow-Point (armor shredder) are loaded.
M44 Combat Revolver: Strong, but low magazine count. Has heavy(stun/silence) bullets, and marksman (high damage/AP) for ammunition variants.
Armor:
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Light:
As the name implies. Less protection, but much less inhibited move-speed.
Medium:
A good all-around armor. Has 3 internal 'spaces' for ammo and such instead of 2.
Heavy:
A heavily protective piece of armor. However, it inhibits move-speed. Expect to be left behind or be fighting for your life if you're wearing this armor and the xenos organize a push.
XENO:
The Xenomorphs. A.K.A species XX-121.
A lot of castes have 'strains' which make their gameplay different from the normal.
T1 Castes:
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Drone:
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Build defenses and structures for the hive. Use pheremones to buff allied xenos in, and out of combat.
Sentinel:
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Use your neuro-toxic spit to slow, daze, knock prone, and eventually S L E E P the marines. Good for capturing marines.
Runner:
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Harass marines by using hit-and-run tactics to wear them down, especially on their back-line and walking routes away from the front-line.
Defender:
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Act as a bulwark between the marines and your higher tiered castes. Soak fire for your allies as they push and / or retreat so that they don't get hurt as badly.
T2 Castes:
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Hivelord:
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Do everything a drone does, but have thicker, more reinforced resin structures. Great for building lots of defenses and structures.
Carrier:
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Carry Facehuggers on you so you can hug the marines while they are down, or when they are dragged to you. Almost ESSENTIAL for getting captures consistently.
Burrower:
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Make traps everywhere that marines step near, dealing MASSIVE acid damage to them.
Make tunnel systems around the map for quick-traveling and escapes.
Spitter:
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Use acid spit to 'tag' an enemy, dealing low burn damage over time.
Use acid spray to deal moderate damage to an enemy, or to remove an active 'tag' from acid spit to deal MUCH more damage.
Good against shotgun users, and for hit-and-runs on or near the front-line.
Lurker:
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Cloak behind enemy lines, pounce, and empower your slashes to deal DEVASTATING damage to marines, and assassinate them.
Cloak gives you more speed boost the further you are away from allies.
Warrior:
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Lunge at marines, stun-grabbing them for 2-3 seconds, slash them, then punch them to deal an almost guaranteed bone-breaking or organ-damaging blow.
Slashing charges up a shield which, when activated, grants you 160 shield permanently until destroyed. Shield does not stack.
T3 Castes:
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Boiler:
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Provide long-ranged artillery acid-gas support for allies. Deny pushes through choke-points.
Don't get ganked or rushed.
Crusher:
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Become a tank for the hive.
Charge in after a short duration, slamming, knocking away, and dealing damage to marines in your path.
Use your other ability to gain a semi-long lasting temporary shield that soaks incoming fire.
Ravager:
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A very combo-oriented caste.
Use empower to gain a Temporary Shield per enemy within sight-range of you.
If you have enough Temporary Shield, your abilities are HEAVILY empowered, as well as your slash damage is increased proportional to your temporary shield amount.
Praetorian:
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A very combo-oriented, ranged caste.
Use your abilities to generate 'stacks' for various abilities to be empowered.
Heavily varied due to the multiple 'strains' praetorian has to vary it's gameplay.
I hope this guide was informative and at LEAST got you started somewhere.
Happy hunting!