Quake Champions, the newest entry in ID Software's classic Quake series, will be a free-to-play game. But players can also buy it if they want. I mean, the game will be basically free-to-play but players can also buy unlimited access to everything the game has to offer for a price.
Free users will be able to play only as a Champion named "Ranger". By playing as Ranger, they can earn "favor" (in-game currency). Favors can be used to "rent" other Champions for a limited period of time or buy "Backpacks", which contain "Rune Challenges". Successfully completing a Rune Challenge will give "reliquaries", which contain Champions and skins.
If this setup seems complicated to you, then you may want to check out the Champions Pack. This pack grants unlimited access to all the Champions the game will have at launch. As of yet, the price of this pack is unknown.
Both paid and free users will share the same maps. As for the Champions, each Champion will have different and complementary skills, which will (hopefully) make sure that the game stays skill-based.
If you want to try out the game before it launches, then sign-up for the closed beta here.