Standard Brush
From ZBrush Info
Standard: The Standard brush is the original basic ZBrush sculpting brush, and when used with its modifiers at their default values, it displaces outward the vertices over which it passes, giving the effect of adding clay to a sculpture. It can be used with all of the various brush modifiers, such as Strokes, Alphas, an edit curve, and so on. Pressing the Alt key causes the standard key to carve into, rather than build up, the model.
