Mobile phones 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 candy-bar or block) This is the most basic style. The entire phone is one solid monolith, with no moving parts aside from the buttons and possibly antenna. To prevent keys from being pressed accidentally while the phone is in a pocket, etc., a "key guard" feature is usually provided, requiring a special key sequence to "unlock" the keys.
Clamshell: (AKA folder) This type of phone 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 have a secondary display on the outside.
Flip: This type of phone is a cross between the Bar and Clamshell types. Most of the components of the phone are in one part, but a thin "flip" part covers the keypad and/or display when not in use. The flip may be all-plastic, or it may contain one or two minor components such as a speaker or secondary keys.
Slide: This type is usually designed similarly to a clamshell, with a large main display and speaker in one half, and the keypad and battery in the other half. But the two halves slide open instead of using a hinge. Slide designs allow the main display to be seen when closed, and are generally easier to open and close one-handed.
Swivel: This type is similar to the slide type, with a large main display and speaker in the top half, and the keypad and battery in the lower half. But the two halves pivot at one point instead of sliding.