In the Inspector window, switch to the second tab, then open the Brush foldout, you will see the properties as listed below:
Mode: brush mode, determine which vertices are affected when painting (see below).
Radius: radius of the brush.
Strength: how fast the terrain elevates when painting.
Erase All: click this button to flush painting data from the terrain.
Currently there are 2 brush mode supported in this version:
Cylindrical: Use a cylinder with infinite height to determine which vertex to modify, that means the Y-coordinate of vertex position will be ignore when calculate distance to the painting point.
Spherical: Use a sphere to determine which vertex to modify, any vertex inside the sphere will be affected on painting.