Mobile phones come in several different physical styles (form factors). While manufacturers are continually coming up with new types of designs, there are three classes used on this site to group form factors:
Block: (candy-bar) 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 Keyguard feature is usually employed, requiring a special key combination to "unlock" the keys. Longer, thinner Block phones are often referred to as "candy-bar" phones.
Flip: This type of phone is identical to a Block type, but has a thin piece of plastic that flips open. When closed, it covers the keys to prevent accidental keypresses. Most flips "flip" down, below the keypad. Some phones flip up, with either separate arms or a clear plastic window allowing the display to be seen when the flip is closed.
Most flip phones have a feature called Active Flip, which means that calls can be answered and ended by opening and closing the flip. On phones with Voice Dialing, Active Flip can also be used to place calls, by opening the flip and speaking a name.
Folder: (clamshell) 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 the display or battery, with the bottom half containing the remaining components. Most Folder-type phones also feature Active Flip, as described above.