Diablo 3 - Runestone System Revealed

The Diablo III site has details of the Runestone system, with several videos and a truckload of screens on offer:

There are more than twenty skills per class, and a player can choose to access up to seven of those skills at any one time. Skills primarily allow characters to perform special attacks and cast spells, but they can also activate defensive abilities, summon minions, set traps, or allow the player to navigate the battlefield through quick travel and escape mechanics. While these skills always present players with plenty of tactical options for defending Sanctuary against the coming demonic invasion, runestones allow you to customize skills by changing the way they look and how they function.