Used for application toolbar actions, menus, and status and notifications. Also used for small (16x16) devices and places icons (folders, usb drives, etc.)
Rely on a distinct shapes instead of fine details to distinguish between them.
Color can be used for distinction (change of state, destructive actions, etc.).
Application icons
Unique and easily recognizable
Respect trademarks by avoiding significant alterations to other application icons
Other icons
Icons of a similar type use a consistent visual language (mimetypes, folders, devices, etc.)
General
Avoid using text in icon designs; it may not scale well to smaller sizes
