Here's some detail about the races:
SpaceMarines: "FOR THE EMPEROR!!!"
- Powerful but few squad members makes your squad last longer
- Customizability to respond to every possible threat. The basic SpaceMarine squad is the epitome of this
- Standard yet got everything they need
- 3 heroes
- Sniper Scouts! 1 shot kills
Conclusion: A solid faction that is easy to learn & use.
Chaos SpaceMarines: "DEATH to the FALSE EMPEROR!"
- Tier 2 Predators! (Anti-infantry tank)
- Defiler! A vehicle that are good at 2 roles but which got moderate HP
- Obliterators! The endgame daemonic infantry that got multiple weapons effective against EVERYTHING!
- Daemon Prince! With a lategame tech, the Chaos Lord can transform into this monster
Conclusion: The basic Chaos SpaceMarine squads are less customizable then their emperorserving brethren and are a bit worse in ranged combat but better in melee. Some units will be a little harder to use correctly then their archenemies the SpaceMarines.
Orks: "Orks are made for figtin and winnin!"
- High damage output from all units
- Large basic squads but cheap
- The Basic shoota squad can get a powerful weapon early and has no setuptime!
- Flash Gitz! Tier 3 ranged troops with superior anti-infantry firepower
- Mad docs! Like SpaceMarine Apothecaries (healers) but these guys can plant bombs
- Killa Kans! Heavy walker vehicle and the only unit in the entire game except relic units to have HEAVY_HIGH armor!
- The strongest heroes DPS wise in melee and ranged combat combined!
- Funny talking! They sound the funniest of all races, that's guaranteed
Conclusion: Some weak units but some really strong ones with strong abilities. The heroes are powerhouses and got great abilities! The Killa Kan got the same armor type as relic units!
Eldar: <Insert random philosophical oneliner>
- Specialized squads (Dark Reapers & Warp Spiders are experts at ranged combat but fight like 5-year olds in melee)
- Fleet of Foot! This ability rewards players who are great at micromanagement and is the reason for Eldar domination
- Abilityheavy race. Their hero the FarSeer are the icon of this with 4 abilities. The 2th hero the Harlequin got 2 but they're very powerful
- Rangers! Infiltrated (stealthed) squad with light sniperrifles that devastates morale and do good damage vs infantry
- WarpSpiders! The Eldar tier 3 infantry squad got powerful anti-infantry weapons and also haywire bombs for use against vehicles and buildings
- 2 ranged damage and armor upgrades that affect ALL Eldar infantry enables a very easy upgrade path for Eldar units
- The Warp Prism is the best Eldar vehicle and except the anti-infantry scatterlaser it got a beam of destruction that fires every 5 seconds which disrupts infantry and also cuts through vehicles and buildings
Conclusion: The Eldar is called Space Elves for a good reason. They're the fastest race with Fleet of Foot and their speicalized squads are very powerful in their roles. They're easy to learn but incredibly hard to master due to the player controlling them needing to micromanage like a Korean StarCraft pro to unleash their full potential.
Tau: "For the Greater Good!"
- Specialized units that are either fast melee or (very) longrange shooters
- The Tau hero only got a ranged attack and the lowest HP but he can upgrade with several powerful ranged weapons to make him effective against everything and he also got great abilities.
- Get increased population limit by building research buildings. Makes it easier for inexperienced players to know when to tech up
- Vespids! This unique jumping hardcapped squad is great support and harassment
- Firewarriors! The basic Firewarrior got the longest rangeattack (with great damage!) of all infantry in the game
Conclusion: Powerful specialized squads and a very powerful hero that requires a great micromanager to really shine.
Necrons: <Silence>
- Unique economy. The Necrons rely only on power and can build as many generators as you want, but the price increases by 20 for each
- Most powerful basic unit. The Necron Warrior are good at both ranged and melee and are tough as nails. They're slow as molasses though. They got 2 upgrades that increses HP and ranged attack. After the 2nd upgrade they're as powerful as tier 3 infantry!
- Can possess enemy units except relic units. The Lord Destroyer can put units in statis and also possess them thus negating the enemy their best units
- Snowballing effect. The obelisks (Necrons listeningposts) raises the population cap, increasing your power for every one you take
- Tombspyders collect fallen Necrons and recycles them to fresh basic infantry of your choosing
- TWO super units and can choose from 3! The relic unit The Restored Monolith and the Necron which can transfrom into The Nightbringer or The Deciever
- The Necron Lord is the most powerful of all heroes. He have a decent ranged attack and a powerful melee attack but his abilities are what really destroys nearly all opposition. He also got lots of HP which increases with every monolith upgrade
Conclusion: At low to medium skilllvls the Necrons are unstoppable since the free basic Necron Warrior aided by the Necron Lord with all his abilites force the enemy to flee or die thus supporting the snowballing effect. A restored Monolith with the entire army supporting it including the Necron Lord is nigh impossible to stop for EVERYBODY! Your only hope is to raid his generators and have large amounts of anti-vehicle vehicles blasting the Monolith down to 2500 HP while having something that kills or ties up his antivehicle infantry.
Imperial Guard: "For Emperor, and Imperium!"
- Large and cheap customizable basic squad, but extremely weak
- Many upgrades for the basic Imperial Guard squad
- Choose from 3 squadleaders to enhance thier effectiveness in three ways
- Can train 9 squadleaders which can be used on their own
- Good selection of vehicles for any situation
- Unique builders that builds and repairs faster then other builders but are hardcapped at 3
- Unique buildingsystem that functions like tunnels in that you can transfer infantry from one building to nearly any other
- Basilisks have the longest range of any artillery in the game and can use special Earthshaker rounds that cost 200/200 each that is absolutely devastating. Basilisks are hardcapped at 3 but one Earthshaker round will destroy an entire infantry squad and three simultaneous Earthshaker rounds will destroy nearly any unit and it will certainly devastate a Necron army camped around its Restored Monolith
- The Baneblade is the second most useful relic unit second only to the Necron Restored Monolith
Conclusion: Infantryheavy army especially in the beginning with lots of tedious micromanaging of attaching leaders to squads. Weak basic unit but large numbers means that the squads effectiveness will go down very quickly. I like them for their tunnelsystem and vehicles though and so should you. It's also the only race with the longest artilley and the ability to fire Earthshaker rounds from their Basilisks mean that they are alone in having an easy time to destroy an endgame Necron army. Their relic unit, The BaneBlade is fun to use and very powerful since it's ranged.
Ok that's my own guide. There's plenty of things I haven't included like Chaos Khorne Berserkers, IG Sentinels and Heavy Weapon Teams but I had to keep the points for each race to like 7 so as to not make too many.