iOS UINavigationBar vs UIToolbar vs UITabBar
The UINavigationBar class implements a control for navigating hierarchical content. It’s a bar, typically displayed at the top of the screen, containing buttons for navigating up and down a hierarchy. The primary properties are a left (back) button, a center title, and an optional right button.
An instance of the UIToolbar class is a control for selecting one of many buttons, called toolbar items. A toolbar momentarily highlights or does not change the appearance of an item when tapped. Use the UITabBar class if you need a radio button style control.
The UITabBar class implements a control for selecting one of two or more buttons, called items. The most common use of a tab bar is to implement a modal interface where tapping an item changes the selection.
To quote big brother:
Tabbar
If your application provides different perspectives on the same set of data, or different subtasks related to the overall function of the application, you might want to use a tab bar. A tab bar appears at the bottom edge of the screen.
A tab bar gives users the ability to switch among different modes or views in an application, and users should be able to access these modes from everywhere in the application
Toolbar
If your application provides a number of actions users can take in the current context, it might be appropriate to provide a toolbar
However that doesn't give you a completely clear application-based decision. The best solution is to look at the iPhone inbuilt applications (Clock and iPod) along with Appstore-approved apps and stick to what is consistent, as that is what the Apple HIG guides and the appstore approval process boils down to.