Stands for "Pixels Per Inch." The resolution of a printed photo is often measured in DPI, or "dots per inch." The DPI describes how many dots of ink the printer prints per line per inch. Therefore, the higher the DPI, the greater the detail of the printed image. However, even if a photo is printed with a high DPI, the detail represented in the photo can only be as high as the PPI.
PPI measures the number of pixels per line per inch in a digital photo. This number is directly related to the number of megapixels a digital camera can capture. For example, the original Canon Digital Rebel is a 6.3 megapixel camera and captures 2048 vertical by 3072 horizontal pixels. Therefore, when printing a 4x6 image, the PPI would be 3072 px. / 6 in. = 512 PPI. That is high enough to print a very detailed 4x6 photo. However, if you were to print a large 20x30 poster image from a 6.3 megapixel image, the PPI would be 3072 px. / 30 in. = 102.4 PPI.
Most modern printers print images with a minimum resolution of 300 DPI. Therefore, if you print a photo with a PPI of less than 300, you may notice the image is not as sharp as you would like. Of course, the detail in a 20x30 image may not need to be as clear as a 4x6 photo. But a good rule of thumb is to keep your PPI above 300 so your prints will look nice and clear.