Leader cards add the extra punch to your Gwent decks, so choose wisely. In The Witcher 3: Wild Hunt, there are 4 leaders for each faction, with 4 more added on with Hearts of Stone expansion and 2 introduced for the Skellige deck in the Blood and Wine expansion.
Be aware that the last leader cards of each faction in the base game can only be obtained through the gwent quest, High Stakes, by beating each opponent.
Each of the four deck types in Gwent has a Leader Card, the commander of the forces represented by that deck. These special cards have abilities that can be invoked during a player's turn, once per match. Used correctly, a Leader Card can cause dramatic comebacks, so it's important to understand what your Leader Card can do and then use that ability accordingly. Each Leader Card has a number of variants that can be earned, found, or purchased, each with special abilities unique to it. The Gwent Card Collection section has a full listing of Leader Cards and their abilities.