The difference between ppi, lpi, and dpi
Lines per inch (lpi) only applies to professional printing. That is, if you are creating a document that will be printed on an offset-litho press. The "line" comes from the old term "line screen" which was used to determine the size of the dots when a photograph was prepared for printing. The lower the number, the bigger the dots. Yes, you get dots from using a line screen measurement. Take a look under a magnifying glass at the photos in a magazine, and in the newspaper. The dots in the newspaper, which can be as low as 85 lpi, are big enough to see with the naked eye. In nice magazines, they are usually 150 lpi, which gives a smoother, sharper, image.
Pixels per inch (ppi) applies to images created to be viewed on computer screens and printed on xerographic printers (like the one in your office, or at Kinkos) and for professional offset-litho printing. Yes, computer screens include tablets and cell phones. The standard ppi for computer screens is 72, and the standard for xerographic printing is 150. Images to be used for offset-litho printing should be 300 ppi.
There is no such thing as dpi (dots per inch). If you hear someone say that, you will either need to clarify if they mean lpi (many old-fashioned professional printers use terms dpi and lpi interchangeably), or if they mean ppi (pixel per inch), which is what most people who aren't familiar with digital graphics mean. I would say that the vast majority of the people who say dpi just mean ppi.
By the way, if you really know what you are talking about, never use the term dpi, ppi, or lpi. Just say, "pixels per inch", or "line screen". If you do that, other people, like me, who know this stuff will know that you know. And we can all relax.
And if all of this is new to you, and you are designing for the web, it's time for you to start thinking in pixels. This link might help.
Posted by Brad Hall