How does cell phone display image or video even before you start to capture? When a cell phone is used to take a photo or capture video the display continuously displays what a camera can see, does that mean the ccd/cmos device is exposed to light continuously? or is it displayed by some other means(with out involving ccd/cmos)? Is the difference between displaying and capturing a video(or photo) is only storage? Is cellphone continuously taking pictures for displaying, even if you don't take a picture?
First off, there is no physical shutter on smartphone cameras, so the CMOS is always "exposed to light continuously", even when the phone is completely off.

The Android camera interface has two modes (actually more, but only two are relevant). One is the "capture" mode, which allows full control over exposure time, resolution, and image processing, and allows to take photos at the maximum quality offered by the hardware. The second is "preview" mode, which produces images at a high frame-rate, but doesn't offer the app as much control over exposure and image processing, and is limited to smaller resolutions, and worse exposure control.

Often, the "preview" mode can also output in a different (YUV) format directly into GPU-accessible memory for efficient display on the screen, while "capture" mode can encode directly to JPEG for storage in main memory.


  Is the difference between displaying and capturing a video(or photo) is only storage?


No, the difference is between the two modes. Capturing a video or photo offers better quality and more control, and can take advantage of hardware encoding to save efficiently to a JPEG or MPEG file.


  Is cellphone continuously taking pictures for displaying, even if you don't take a picture?


Yes, while the camera app is open. When you take a picture, the preview mode stops while the picture is being captured. When you're not using a camera app at all, the camera module is powered off.