Considering the meaning of “Label decoration”, you won’t ever have a formal description of all the icons in a project Explorer: it depends on the plugins you have installed.
A plugin like EGit for instance will modify those icons with its own list of decorators:
As Harry Joy, mentions in the comments, the CTRL+N “new project” dialog is the closest way to list all of those icons.
It can be helpful though to know about the “User Interface Guidelines” for Eclipse to grasp their more general meaning:
For example, in the style section:
The two dominant colors, blue and yellow, bring harmony to the overall presentation of the user interface.
Themselves complementary, blue and yellow form a base on which to apply accent colors
Brown is used less than the other colors mentioned, but it is generally associated with specific types of objects: the Java “package”, “bundle”, and the “Enterprise Java Bean (EJB)”.