Well, bangs are defined by specific modules loaded. What the specific bangs do is outlined in the module's docs. Bangs, generally, need some kind of event to fire. Events are also defined by the modules loaded, as I'm sure you've seen inside the module docs. Like OnLeftClick !bang, OnEnter !bang, *Popup "Name" !bang (left click is automatically an event in that case), and so on.

In some cases, you don't need a bang. For example, if you want to open a web browser with a label button, you can have:

LabelNameOnLeftClick "$Browser$"

The same button to recycle Litestep:

LabelNameOnLeftClick !recycle

There are a bunch of predefined variables inside the \personal\evars.rc file.

Hold Shift+CTRL (edit - oops, wrong keys originally) and press F1. That will take you to some docs, which are outdated, but still contain useful information. You've already found the wiki, which is a *** to navigate (imo), but also has useful info.