These four terms have caused confusion in the prepress, design and printing industry for as long as they have existed. Often they are used interchangeably, but each one does in fact have its own meaning.

Firstly, the acronyms stand for the following:
DPI, Dots Per Inch
LPI, Lines Per Inch (often called Line Screen)
PPI, Pixels Per Inch
SPI, Samples Per Inch

PPI and SPI are the easiest to define, as the meaning almost is interchangeable. PPI refers to the numbers of pixels an image has per inch, in both the X and Y direction. So 92PPI means there will be 92x92=8462 pixels in a one inch square. SPI means exactly the same thing, only PPI is used to refer to an image on screen, whereas SPI is used when scanning a document, i.e. taking a 'sample'...

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