Smartphones typically have an LCD graphical screen with touch input. Touch is used to take user input via windows controls such as menus and command buttons. A SIP (Software Input Panel) may also be used am on-screen keyboard. Alternatively, some phones use a small physical keyboard. Some use both. Most touchscreens are single input, that is only one point of contact can be deciphered. Multitouch is becoming a required feature. Multitouch allows not only simple zoom and rotation of images, it allows quite complexes gestures. Windows 7 supports Multitouch with suitable hardware.
A touchscreen has two uses. One is as a pointing devices, that is as a mouse. The other is to action gestures such as click, double click etc, It can be used with a finger or stylus.
Windows CE can support an LCD panel with a touchscreen; it has for many versions. Windows CE 6 R3 supports single finger gestures. My HTC phone (HTC HD2) support Multitouch, event though it’s Windows Mobile 6.53, which is CE 5 based, doesn’t intrinsically support it. My guess is that the next version of CE will support Multitouch. Windows Phone 7 requires Multitouch, but that is is based upon CE 6 R3 (my guess). ????
The task for a touch screen is to determine the X-Y coordinates of the touchpoint (or touch points).
LCD = Liquid Crystal Display
Standard LCDs use a matrix of polarization cells that electrically control whether light is transmitted. There are reflective and transmissive LCD panels. The reflective type use the least energy as they use the reflection of ambient light (light) (or non-reflection: dark) to create an image. They are quite viewable in sumlight, as per a digital watch. Transmissive LCD panels use a backlight to generate light that is either transmitted or not. Colored displays have a combination of primary colors. Transmissive LCD panels require more energy, but not as much as a Plasma display!
New LCD displays are based upon LEDs and are much more efficient.
Organic Light emitting Diode (OLED),
CE Touchscreen driver http://220.127.116.11/web20/eGalaxTouchDriver/ceDriver.htm