To submit a form (in case type="submit" and the command button is contained within a form component) or
To redirect the navigation to another page or
To execute a portion of code when the user click on it
A command button tag is "commandbutton"
i.e.
<comp:commandbutton name="submit_info" type="submit" caption="Submit your info"/>
A command button has a caption, which is the text shown within the button. A command button also has a type (which is the same type as set in a html button) A command button has a src property to set the location of the image to show instead of the button itself.
It's important to note that a command button of type="submit" and embedded within a form component will raise the submit event associated to that form when clicked. Of course, it will also raise the click event, but the better way to associate code to a form submit is within the submit event.