Pixels

“Pixels” comes from ‘picture element’ and they are what make up graphic images. Each image is made up of thousands or millions of pixels that store information called 'bits'. Bits define the intensity of each pixel and the colour modes are RBG, CMYK or Greyscale. 
RBG - Red, green blue
CMYK - Cyan, magenta, yellow, black/key
Greyscale - different shade of grey

Although they seem to be connected when viewing graphics on digital devices, the pixels are actually lined up perfectly in rows and columns. If you zoom in enough you can see each individual pixel, for example:

The standard measurement for pixels is DPI (dots per inch) and the higher the DPI, the higher the quality will be because it has better resolution. For example if you are creating an image and you set the DPI to 72 which is the DPI setting for graphics on the web, there will be 72 pixels in each square inch of that image and therefore if you zoom in you will be able to clearly see the pixelated image. On the other hand if you set the DPI to 300 which is the DPI setting for images being printed, there will be a lot more pixels in each square inch and the image will be much clearer.

72 DPI

300 DPI