What is a Dropdown List Button?
A dropdown list button is a button that, when clicked, makes a dropdown list appear under it. This dropdown list allows the user to select items (in almost all cases, only one single item). After an item is selected, the dropdown list disappears, leaving only the button visible. Normally, the currently selected item is displayed inside the button, so the selection is always visible even when the list is closed. In most cases, a dropdown list will feature items with textual names, but they could be simple icons with tooltips, and in such case the button will change its icon to match the currently selected item's icon.
Dropdown list buttons are normally called simply "dropdown lists," however the "dropdown list" is technically the list that appears, and not the element that is visible when the list is hidden. More importantly, there are other elements that feature dropdown lists, such as combo boxes.
Dropdown list buttons are similar to combo boxes, except that in a combo box you type in a text box the item you want to select, and, in some cases, it's possible to type an item that doesn't yet exist to create it. In both cases, it's possible to navigate the items on the dropdown list with the arrow keys, and select one by pressing Enter. While the dropdown list has keyboard focus, it's also generally possible to quickly skip to an item by typing its name. For example, if you click on a dropdown list button, and a list appears with the name of all sorts of fruits, you can press the letter P to quickly select the first item that starts with P, e.g. "Papaya."