Resolution, DPI & Image Size
The resolution of a raster image is its size in pixels, for example 3000 × 2000. PPI states how many image points are output per inch and, only in combination with the pixel count, determines how large an image can be printed without looking soft. DPI strictly refers to the raster dots of the output device and is considerably higher; in everyday use the two abbreviations are often treated as interchangeable.
In practice
For screens the DPI value stored in a file is irrelevant; what counts is the pixel count relative to the display area. For print the rule is that an image cannot become sharper after the fact: upscaling a file only produces larger, softer pixels. The most common mistake is reusing images from social media or your own website in a brochure, since web images are deliberately downsized. Always request the unedited originals at full size after a photo shoot and archive them, not just the cropped web versions. Conversely, huge print images should never go online unchecked, because they cost loading time and therefore visibility.