The Pixel Per Inch measurement, or PPI for short, is a measure of screen clarity. The higher the value, the more pixels that can be found in one square inch of a screen, and the clearer and sharper the picture. Check out Wikipedia to see how this value is computed.
We’re finally starting to see higher resolution screens, which means that no longer do images and graphics appear grainy, and we can browse the internet with integrity closer to what you’d experience on a desktop.
When the iPhone was first announced with a 160PPI screen, it was a big deal. Most phones prior had been sporting at best QVGA resolution (the HTC Wizard, T-Mobile Dash, Motorola Q, etc). Today, a 160PPI screen is middle of the road in terms of what is available.
Below is a PPI comparison chart with some of the most popular phones today. The phone with the highest PPI screen is the upcoming Sony Ericsson XPERIA X1, which will sport 800×480 resolution in a 3.0″ screen (ditto for the Toshiba G910 – too bad we really didn’t like it). Such high resolution is great when using full screen programs like Opera, but it’s not utilized well in the rest of the WinMo UI because the top and bottom bar still uses up a huge amount of screen space.