The term pixel is actually short for Picture Element. These small little dots are what make up the images on computer displays.
When you create an image for your website, knowing the resolution of that image is important. If an image is greater in width than a user’s screen width, then the user will not be able to see the whole image.
The PPI (pixels per inch) or DPI (dots per inch) of a computer display is related to the diagonal size of the display in inches and the total number of pixels in the horizontal and vertical directions.
Smaller devices like an Iphone 5 can display a high resolution similar to a desktop computer by increasing the number of pixels per inch. A computer monitor that has a resolution of 1680 x 1050 and is 22 inches measured diagonally will have 90 PPI. A mobile phone with a resolution of 1680 x 1050 and a 4 inch display will have 495 PPI.
Images that will be displayed on a website are usually saved at around 72 PPI. This is because many monitors display close to 72 PPI, and 72 PPI images load faster; if you have a high density or high PPI image it will be a bigger file size and load slower. However if you want to print a picture you should save it at 300 PPI.