Digital images are made up of pixels. The image itself is a matrix of
so many pixels wide by so many pixels high.
A 6 megapixel image, for example, roughly has a width of 3000 and a height
of 2000 pixels. PPI stands for Pixels Per Inch. PPI then is used to describe
how densely the pixels that make up an image are packed within an inch.
A landscape requires more PPI since it may be very detailed. By contrast,
a typical portrait can get away with less initial PPI. Also, not all cameras
produce equal quality images.
stands for dots per inch. Confusingly, it often is used interchangeably
with PPI to describe the resolution of digital images/ scanners (which
produce digital images).
This is why there is much confusion since DPI refers to the resolution
of an output device such as a printer. With a common everyday inkjet printer,
DPI represents how many dots of ink are placed within the span of a linear
inch on your printed image.
Such printers use a limited number of coloured inks to produce the millions
of colours possible in a photograph. Printers vary in respect of how many
colours they employ. Some have three, four or even six or more. To make
allowances for the limited number of colours, each pixel of the digital
image is made up by mixing many tiny dots of ink together.
By way of an example, a 2400 DPI printer has 2400 dots of ink packed together
within an inch. If an image were 300 PPI, your printer would use about
48 dots to make up one pixel from the digital image.