Two Common Resolutions:
An image file imported into an application will display with a certain resolution, i.e., a certain number of pixels per inch. There are two default standards that many word smithing and graphics editing applications use:
72 dpi
This resolution comes for typesetting where there are not exactly 72 points per inch. With the advent of computers and computer graphics, many standards use an exact measure of 72 points per inch [DeskTop Publishing points -or- DTP Points]. There are a few purists for whom the difference is important, but for the most part, nobody cares!
What people do care about is the quality of their images when they print the image. As a general rule, 72 dpi is a poor choice.
96 dpi
It took me a long time to figure out 96 dpi—why not 100 dpi—the math is easier? Well, back in the ancient [more than a year ago] history of computers, for one type of fairly standard, monochrome, monitor, there were 96 pixels per inch on the screen. So why not make it a standard—paying no attention to different sized monitors with different resolutions. Whatever the reason, it stuck and so we are stuck with it.