There are four levels of rarity in Sorcery, described below. Note that Avatars do not have a rarity listed on their typeline and thus don't have any rarity during gameplay, but you still only have one Avatar!
The lowest level of rarity. You may include up to 4 copies of an ordinary card (by name) in your deck.
The second lowest level of rarity. You may include up to 3 copies of an exceptional card (by name) in your deck.
The second highest level of rarity. You may include up to 2 copies of an elite card (by name) in your deck.
The highest level of rarity. You may include only 1 copy of a unique card (by name) in your deck.
We don't talk about curios!