What attack does the Wizard do when attacking with a spell while out of Arcane Power?
Yes, that is the basic weapon attack. When you start out a level 1 character, it's the skill that is on right mouse button. With wands it's a small projectile, with swords you would run up to the monster and slash it.
You can also set this skill if you are in elective mode, and assign a skill to a hotkey when it is already assigned. For instance, if you have Magic Missle assigned to left mouse, and decide to remap it to right mouse, you will get the default weapon attack set to your left mouse button.