Knowledge is power, but with power must come control. In Druids, the novices of the Stonehenge Academy must collect experience points in various knowledge domains, but you don't want knowledge from just any domain because if you enter a domain not intended for you, then you lose all knowledge previously gained that round.
In more detail, each player is dealt a hand of cards, with the deck containing cards in five colors, numbered 1-12 in each color. Players then play a trick-taking game, and whoever wins a trick must place the cards sorted by domain (color) in front of themselves; if they receive multiple cards of a color in the same trick, the lowest-valued card must be placed on top. The tricks continue until either all cards have been played or one player collects cards in all five colors. In this latter case, that player receives negative points for what they've collected while everyone else scores positively for their topmost card in each color. (If all the cards have been played, then all players score.) Whoever has the most points after five complete rounds wins!
In addition to the regular cards, Druids contains special cards that allow a player to avoid an unwanted trick or remove cards of one color from a player's holdings.