The toolbar allows quick access to some of the most useful features of IrfanView.
Turn the toolbar on by using the View Menu and selecting the Show/hide toolbar option. Turn the bar off by again clicking on View and Show/hide toolbar. There is an image of the toolbar here.
In the Properties->Toolbar dialog, you can choose different skins (button designs) or add/remove toolbar buttons.
More toolbar skins are available on the IrfanView homepage.