Champion skin (or just skin) refers to the color scheme or appearance of a champion. In League of Legends, most skins can be bought from the Riot Store with Riot Points, while others are or were available only from special events or promotions. Some of the skins in League of Legends also give champions different particle effects, quotes, and sound effects. Every skin has its own splash art that is displayed in champion select by clicking on the thumbnail after selection.
To see all current and old splash arts, visit this gallery.