See the Brush Display options.



Paint options.

The Weight Paint Options modify the overall brush behavior:

This option limits the influence of painting to vertices (even with weight 0) belonging to the selected vertex group.
X Mirror
Use the X Mirror option for mirrored painting on groups that have symmetrical names, like with extension ".R"/ ".L" or "_R" / "_L". If a group has no mirrored counterpart, it will paint symmetrically on the active group itself. You can read more about the naming convention in Editing Armatures: Naming conventions. The convention for armatures/bones apply here as well.
Topology Mirror
Use topology-based mirroring, for when both sides of a mesh have matching mirrored topology. See here for more information.
Unified Settings
The Size, Strength and Weight of the brush can be set to be shared across different brushes, as opposed to per brush.