In reality, the screen resolution at a standard inch is still about 72 PPI, the same as a Mac. Working with screen-based Web or multimedia images, DPI has no meaning whatsoever. If you scan an image at 50 dpi or 600 dpi and it is 100 pixels wide, it will be 100 pixels wide on any screen regardless of its resolution. Of course, if it is on a 640 x 480 screen it will occupy a larger area of the screen real estate than on a 1024 x 768 monitor. If you think of screen images as being like mosaic tiles where every tile is a different colour but fixed in size, you will get the idea.
Pixels are just mosaic tiles made from glowing phosphors or LCD elements on a screen.
Research shows that more than half of all Web surfer use an 800 x 600 pixel monitor followed by 1024 x 768. 16-bit colour also accounts for more than half of all surfers. Web designers use more highly specified monitors and computers but should make sure that their pages work for the majority of surfers at 800 x 600, 16-bits. Using type is more confusing. 12 Point type on a Mac is about 12 pixels high, but 12 point type on a PC is a third bigger - nearly 16 pixels high.