There are command and menu buttons.
You click on a command button to perform or activate the command. You click on a menu button to expand the menu, to show the items within it.
Buttons can appear in toolbars. They can also appear in menus (so you can have a menu within another menu).
Some buttons are combined command and menu buttons.
Some buttons have a lasting effect. For example you start the Polyline command by clicking its button, and the command stays active until you de-activate it (by right-clicking for example). While the button is active, this is indicated by its appearance; by an orange background, for example.
You can:
Opt for tooltips to be displayed when you rest the cursor on a button, showing the button's name.
Opt for the shortcut key of the button's command to be shown in the tooltip.
You can change the appearance of a button in these ways:
Display the button as text (the name of the command or menu), or an image, or both.
Customise the image. You can change to one of a pre-defined set of images, or create an image yourself.
Reset to the default image.