Nilfgaard's Empire
This faction is centered around white and black colors with the Human-Soldier type being the most present card in it.
The core mechanics that are present in Nilfgaard are Strength in Numbers and Multitude.
Scoia'Tael Rebels
This factions breaths around green and black colors with the Elf-Rebel type being the most present card in it.
The Scoia'Tael share their core mechanics with Nilfgaard but have a different playstyle per se.
Witchers
Witchers come in all shapes, so you can have a rainbow army of machine-killing monsters at your service if you want.
Their signature mechanics it's called Contract and they can have sometimes other faction's mechanics.
Wizards
Wizards are unsurprisingly represented by red and blue colors, even if there are some examples of UB and RG mages around.
Their core mechanic it's called Sources of Power and might be one of the most straightforward and powerful abilities in the set.
Monsters
Monsters come in all colors and shapes, they mainly work on Multitude and Strength in Numbers mechanics and have a Sliver-like feeling.
They are an easy to play hard to master factions since you probably need 3 to 4 colors to get the most out of them.