The button's type attribute determines the semantics of the other fields, so we'll first take a look at the possible values:
- action: This makes the button call an action as defined in the ir.actions.* namespace. The name attribute needs to contain the action's database ID, which you can conveniently have Odoo look up with a Python format string that contains the XML ID of the action in question.
- object: This calls a method of the current model. The name attribute contains the function's name. The function should have the @api.multi signature and will act on the currently-viewed record.
The string attribute is used to assign the text the user sees.
There used to be a third value, workflow, which sent a signal to the by now deprecated and removed workflow engine. If you come across this when migrating code from older versions, you'll probably have to replace it with a method call.