Pixel (picture element) is the smallest area of information within a color grid used to form a picture. Several may be measured linearly within an inch. Thus, 72 PPI (pixels per inch) is what most CRT computer screens traditionally used. Technology advancements have brought about many displays in excess of 100 PPI -- forgoing the one-to-one ratio with rulers.
In the infancy of digital camera development, the ability to capture 640 x 480 pixels was a milestone - a million pixels or one megapixel. Now, multi-megapixel cameras are in the mainstream. Before reaching a point of diminishing returns, more pixels roughly equates to higher resolution and image detail. Regardless of the number of pixels, clarity can be affected by image noise and compression artifacts.
Offset printing photographic images in magazines or glossy brochures typically call for 240 to 300 PPI at final print size. Using inkjet printing, Reactive Imaging can reproduce large high quality posters with 180 to 240 PPI at final print size. Acceptable results can be achieved from photos with a resolution as low as 100 PPI distance viewing. Higher resolution images than these generally just increase processing time with no discernible improvement in print quality.
If preparing documents at a smaller size, be certain to consider enlargement when determining photo resolution. For a layout prepared at half size, images should be 200 to 480 PPI.
A related term, DPI (dots per inch), is often interchanged with PPI. However, in digital imaging, DPI (or LPI for lines per inch) is a printing term for rows of dots that simulate continuous tones with few colors (usually 4). A general rule is provide twice the amount of PPI than what the offset printer will use for DPI. For standard printing at 150 DPI, images are usually provided at 300 PPI. Less PPI is usually required for stochastic screening. Printing stochastically at 200 DPI can appear as smooth as 450 DPI.