Understanding the Differences
DPI - which stands for dots per inch - measures the spacial dot density, or individual dots that can be placed in a single, straight line in the span of a single inch. PPI - which stands for pixels per inch - measures the dot density of individual dots that can be placed in a single, straight line in the span of a single inch. So they both mean the same thing, which means that they must be synonymous, right? Actually that's a common misconception that even some of the most seasoned graphic designers still make. No, DPI and PPI are not the same thing, and here's why.
DPI refers to physical dots, such as those created by a printer. The more dense the DPI, the more vibrant the colors and more seamless an image. When printing something in high-quality, you generally aim to print somewhere around 150-300 DPI, largely depending on the end use of your printed media. However, it should be noted that because of the limited colors of printer ink, a 5x5 dot image will usually take more than 25 dots to reproduce. This is largely because your printer must take the time to mix the four colors included (magenta, cyan, yellow, and black) to create the colors that we see in things like photographs. A common misconception is that DPI is related directly to image resolution, but it is actually only related tangentially - it is more of an independent factor than most people think.
PPI, however, refers to the amount of pixels displayed within an inch on a computer screen. For years, the PPI standard had been 72 pixels per inch, or 72PPI. That meant for every inch, you could lay a row of pixels end-to-end and fit 72 of them inside. These days, there are monitors and mobile devices that are capable of displaying much greater PPIs. For example, my iPhone 4 has a 326PPI display, and my computer monitor has a 102PPI display. What does this mean for the average computer user? Here is where there is a direct correlation between resolution and PPI. The larger your screen resolution, the higher your PPI. A monitor that uses 800x600 mode will have a lower PPI than a monitor using 1024x768 mode. The lower your resolution, the less pixels fit on your screen, making windows appear larger and your workspace more cluttered. The higher your resolution, the more pixels per inch, and your workspace will feel much less cluttered, which is great for people who work in UI-heavy programs like Adobe Photoshop. Higher PPIs also have the added bonus of producing, sharper, clearer images.