Here is a video explaining the difference between dpi and ppi.
Dots Per Inch (DPI) is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch (~2.54 cm). DPI is used to describe the resolution number of dots per inch in a digital print and the printing resolution of a hard copy print dot gain, which is the increase in the size of the halftone dots during printing. This is caused by the spreading of ink on the surface of the media. For a more detailed look visit this page.
Pixels Per Inch (PPI) is a measurement of the pixel density (also known as resolution) of devices in various contexts. For the most part this refers to displays, scanners, and digital camera image sensors. PPI is defined as the horizontal or vertical density (for square pixels) as those are the same but the density on along the diagonal is lower. Square pixels are the norm (otherwise those densities would be different). For a more detailed look visit this page.