You can find much detailed looks at each of Combat Cards’ factions in the early Dev Blogs at but here’s a very quick overview:
Space Marines – A large faction containing all the Space Marine Chapters. Most Space Marine cards can make at least two of the attack types, meaning they’re flexible in battle, but tend towards higher points costs.
Chaos – This faction includes Chaos Space Marines and Chaos Daemons, so is extremely flexible, but with a tendency towards melee and psychic attacks. Chaos decks put the most emphasis on their Warlord, so should be built around this.
Necrons – Necrons have no psychic attacks, but can do well in both ranged and melee combat. Their speciality is survival, outlasting other factions with the number of cards and Wounds they can bring to battle.
Orks – Ork decks are usually focused on melee combat, but this means thought needs to be given to how they’ll deal with opponents who are even better at melee combat than they are! Careful card deployment can help with this.
Servants of the Emperor – Because it includes all the Imperial armies other than Space Marines, this is the largest and most diverse faction. Decks tend towards melee or ranged attacks; and can easily fill all bodyguard slots with cheap cards.
Aeldari – Contains the Drukhari, Craftworlds, Harlequins and Ynnari, so Aeldari decks are highly flexible. They’re also one of the more ‘psychic’ factions, meaning you can balance all the attack types or focus on one.
Tyranids – Favour melee attacks, but can create decks around ranged or psychic combat. Decks can be built around a horde of cheap cards or a few, high Wound monsters, but a combination of both offers the most flexibility.
T’au – Favour ranged attacks (and are probably the strongest faction in this area) but have no psychic cards at all. T’au decks tend towards a few very high Wound cards supported by cheap infantry and Drones.