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The light strikes a digital sensor array, instead of a piece of film when you take a snap. These digital sensors are computer "chips". These are called like CCD, CMOS, Foveon etc. They take the place of a piece of film that must be moved across the focal plane of the camera.
The digital sensor is made of millions of tiny sensor points called "pixels," which is short for "picture elements." They are laid out in an array with rows and columns, like in a computer spreadsheet or wall calendar. For instance, my camera has an array of sensors in its CCD that is 3008 horizontally, and 2000 pixels vertically (3008x2000).
If you do a simple mathematical formula on the pixel array size you will come up with the "Megapixel" rating of the camera. This is the number that most manufacturers use to sell the camera. The simple formula 3008x2000 = 6,016,000 shows that my camera has over six million pixels, or is a "six megapixel" camera.
When the image strikes the sensor, it gets all those megapixels excited. First the image goes through color filters above the individual sensors. The sensor converts the image from light waves into an analog electrical signal. The analog signal is then run through an analog to digital converter (A-D Converter), where it becomes a pure digital signal.
Then it is again put through a series of electronic filters that adjust the white balance, color, and aliasing of the image. Next a compression cycle makes the image as small as possible by dumping unnecessary pixels, for more efficient storage. Now the camera has a nice compressed, filtered, digital signal representing your image.
The image is then transferred into a temporary storage area inside the camera called "buffer memory," or simply the "buffer." When the buffer is full, the image is written out to your storage media, such as a memory card. The buffer size in the camera is an important thing. It tells how many images you can take in quick succession. If you have a tiny buffer in your camera, you will have to wait a bit after you take several images.
In fact, the main thing that drives the cost up on digital cameras is the number of megapixels, and the size of the memory buffer. Most cameras have a reasonable amount of both, so you needn't worry. Even if you can only afford a very inexpensive digital camera, you will still have nice images, you just might be limited in their maximum size on photo paper, and will have to wait a bit when taking images quickly. Almost any digital camera is capable of taking pictures for display on the Internet, or for sending across the Internet as email. Images on the Internet are very low resolution -- about 72 to 100 dots per inch -- so any quality digital camera will be capable of making beautiful images for display there.
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