Mobile phones can come in several different physical styles (form factors). While manufacturers are continually coming up with new types of designs, there are several common categories used on this site to describe form factors:
Bar: (AKA block or monolith) This is the most basic style. The entire phone is one solid monolith, with few or no moving parts aside from perhaps buttons or a switch. Most smartphones come in this form factor, and it was a common style for feature phones (non-smart-phones) as well.
Foldable: A style of smartphone with a flexible display that lets it change size/and or shape, typically folding in half.
Clamshell: (AKA flip) A common style of feature phone (non-smart-phone) that consists of two halves, connected by a hinge. The phone folds closed when not in use. The top half usually contains the speaker and display, with the bottom half containing the keypad and remaining components. The main display and keypad are protected when closed, although many phones of this type have a smaller secondary display on the outside.
Feature phones came in several other styles as well, including slider, sideways slider, swivel, and a type of flip phone that was just a bar phone with a thin flip cover over the keypad.