A slide phone is a physical mobile phone design in which one section of the phone is used for the display, which slides apart from the other section, which is the keypad.
The key advantage of a slide phone is its ability to accommodate a large screen size and large keypad area without consuming too much outside phone space, particularly when a phone is in a retracted state. When retracted, the keypad remains hidden behind the display area. A user simply slides out the keypad as required. Some keypads slide along the phone's vertical axis, while others slide along the horizontal axis. The keypad has either a standard or QWERTY layout.
See more at "Form factor of mobile phones".