For Odoo v7 it will be context.get('active_ids')

For Odoo v8 it will be self.env.context.get('active_ids') 

Both will return a list of selected ids (if in tree view) or a single element list if in form view and used an action button.

I subscribe to the questions.

How pass to qweb report ids of all records from tree view, because selecting on screen gives fixed amount (typically 80). I don't want to change this default setting for e.g. for 100000 ;-)