A activates looting, bonfires, player comments etc.
D-pad cycles between alternative items for that slot as defined in the equipment menu.
performs a regular attack when moving normally.
Left trigger performs a shield attack (for want of a better description).
Start opens the in-game menu for items etc.
Clicking Right stick centers the camera behind the player.
Moving Left stick moves the player with two different speeds: walking and running.
I can't seem to find a reference for all the mechanics of Dark Souls as played on PC (specifically Prepare to Die Edition on Steam in case that makes a difference).